Gonna see how much scattered info on the super giant overhaul mega patch I can condense into a single place, to make referencing information easier. If I missed anything, let me know.

The Dev Blogs

Dev Blog 1 - Announcing Our Biggest Game Systems Update Ever!

The first dev blog that announces the new "Talent" system.

Highlights:

Every hero is getting a Quality of Life update as part of the new systems.
Powers no longer use Power Points and will have the former benefits of maximum Power Points incorporated directly into them automatically.
Items will no longer give Power Ranks. What will replace this is still to be announced in the next Dev Blog.
All heroes will now be restricted to a single 8-slot Power Bar for slotting active powers.
Travel powers have been removed as slottable powers and will now have to use the new hotkey.
The old Crowd Control cleanse powers have been removed and incorporated into "Movement" powers like dashes and teleports.
Talents. Heroes now have 5 sets of talents that unlock as they level. Each set lets the player pick one of three choices that will alter powers or give other bonuses.
Traits. Heroes will now have an offensive and defensive "Trait" gained at 1st level. Through their Offensive Trait, all heroes have two Attributes selected that give them a scaling damage bonus based on how much of that Attribute they have. Defensive Traits will contain the Total Defense/Deflect/Dodge multipliers and +8% Damage Reduction affix currently found on defensive passives. Both will also contain a "flavorful" affix to make the hero more unique. Some heroes will have a third "Mechanic Trait" that explains their secondary resource or other complex mechanics unique to them.
Attributes are being overhauled "with the goal of making all Attributes useful for all Heroes." More details in a future Dev Blog.
Contains examples of these changes, including new powers for Luke Cage, Loki and Human Torch.
Official Hero Power Update FAQ. Check here first for truly official answers. @AgentVapor went and posted it while I was still working on mine. Totally ganked.

Dev Responses, Clarifications and FAQs

Power Points and Power Bar

Ultimate Powers will not be changing, will continue to have 20 Ranks and Signatures will continue to receive bonuses from Ultimate ranks. [source 1, 2] Items that add Ultimate ranks will still do so. [source]

Power Trees, Talents and Traits

Choices are mutually exclusive -- you may only pick one from each of the five sets -- and can affect categories or sets of powers. [source] (Example of specializations converted to Talents using Cable)
Talents replace all former passives, toggles and specializations. [source]
All existing powers have been "slid over" to fit into two power trees instead of three, freeing the third panel for Talents. Very few if any powers have been removed outright. [source]
@Doomsaw says the new changes look "awesome". [source]

Summary of Asros's "Have Questions about Item Changes With Talents?" Thread

Lots of pimpin' for the upcoming Items and Difficulty Slider Dev Blogs. You our favorite hypeboi, @Asros. <3
"We have reviewed every unique in the game and brought them to a relative budget range within each other. Almost every unique will get some kind of buff to make them better at what they do. A lot of this is by adjusting attribute allocation to work with the new attribute changes." [source] Yes, "Every Unique." [source]
Power Ranks will be replaced with new properties, mostly attribute bonuses. [source] This includes Cosmic items and their +1 ranks. [source]
Loot tables are receiving a massive overhaul. Pet Cybernetic requirements will be lowered to match the move towards dropping "less undesirable loot". [source]
"The attribute system is changing so that while the attribute still gives specific stats, you will almost always want to gear for your hero's primary attribute, which may not be Intelligence for summoners." [source]
Items that can be slotted to the Power Bar and activated will be losing this functionality. The only exception is the Ziggurat of Kargul, which is still to be determined. [source]
@jba got to have a cookie. Everyone without a cookie should glower with jealousy. [source]
The fate of Story Mode Health and Spirit bonuses is still to be determined. [source]
"We have work being done to make RIF better, including allowing it to impact boss specific chase item drop rates that are either coming with this system update, or shortly after. We want to make RIF and SIF great again!" [source]
Move Speed will not be removed from existing items (to prevent rerolling), but future items will no longer receive it as an affix. [source]
No plans for "set items". [source]
The Medkit button and affixes will continue to work identical to how they work now, but will no longer require Medkits in your inventory to work. [source]
"We aren't anticipating the changes to items to cause massive rerolls on items. Power points(the main things removed) are controlled from different item seed than other affixes. I cant promise that none of your items will be re-rolled, but I am very confident that it wont be a problem. However, new attributes being added to items will not inherit old power point rolls and will be given their own new percentages." [source]
"A goal of ours going forward is to reduce the number of affixes on items, especially slot 1-5." [source]
Medkit related achievements are moving to the "Feats" section with other defunct achievements. Do them now or never. [source]
"Current plan is for this month to finish strong with the Agents of SHIELD event, then Luke Cage, then October will be pretty quiet while we work with you all on test center with these new changes. After that, something exciting in the works, but cant divulge yet." [hype source]
"We have designs to make all of the 'level up' artifacts much more viable while leveling and fit builds way better. I'm not sure if that will make it in with this system update or not, but it should within a few months or so." [source]
"No uniques are being removed. Actually, Luke Cage's Disco Shirt is actually becoming a real unique :o" [Sweet Christmas!]
Uniques with high power rank rolls will not necessarily get good Attribute rolls. "Power point affixes and attribute affixes are handled totally differently." [source]
Champion commendations will one day totally be useful. [source]
Niche items like the Doomsaw are anticipated to continue filling those niches instead of being overhauled. In general, Any Hero Uniques won't see "drastic changes." [source]
"We will be removing flat size increase or decrease from items. Our modelers and vfx artists have begged for this for a long time, as it makes building visuals, costumes even more time consuming to have to account for different sizes. We plan to, for the most part, move these instead to procs on the item to grow/shrink for a few seconds instead of being huge/tiny all the time. We know some players really enjoy these affixes and sympathize, but the back end downside is too great." [source]
The "customizable uniques" system has ceased development for the time being. [source]
"We are looking in to Power Duration and agree with your assessment that the amount available can break a lot of things. When we get closer to TC, we will have more information on our plans." [source]

Legendaries

Possibilities for new Legendaries. [source]
Norn Stones will be updated. [source]
Other existing legendaries will not change in this update. [source]
Movement-based Legendary? ";)" [source]

Starktech Cubes

Starktech Cubes will be going away and existing ones will become Ancient Forgotten Devices to be traded to Clea. A few new things may be added to the vendor list. [source]
The new items will not be announced ahead of time. [source]
Engineering isn't complete, but the goal is to count both slotted and unslotted cubes. [source]
The previous hinted update to them has been scrapped for now. [source]
There are no plans to bring the cubes back. [source]
Exchange rate is still to be determined. [source]

Stuff Coming Later (Not This Update)

The free blessing swap will be extending "a month or two post this update". A cost will be added later. [source]
Changes to the way costumes are handled/slotted is in the works, but not until after the current system updates. [source]
"No changes to costumes/cores/affixes in this update. But there will be some in an update soon after. Will discuss more as we get closer to that." [source]
Relic changes are not a part of this update, but likely afterwards. [source]
The ability to reroll single affixes instead of an entire item is being worked on, but will not release with this update. "It also won't be cheap to do." [source]
There are plans for a system to differentiate between costume core rarities in development. [source]
"We have some plans to simplify the rune system and clarify a lot of it. It wont happen for the system update, but shortly after, if all things go according to plan." [source]

Other Stuff Called Out As Not Changing This Update

No new or altered Runewords beyond Attribute and Power Point changes, but the itemization updates will open up opportunities for more later. [source]
Crit bonuses "will receive a balance pass of sorts, but will remain mainly unchanged." [source]
No changes to the Summoned Ally Damage affix. [source]
No changes to Team-Ups. [source]
No changes to Eternity Splinters. [source]

And if all else fails...

Miscellaneous

Prestige and Omega progress will not be lost. [source]
Prestiges higher than Cosmic are being considered, but not until after the Talent launch. [source]

Heroes

Cable

Used as an example of how the old Specialization system is being converted to the new Talent system here. For example, he can now take all three "keystone" powers (Particle Accelerator, Psimitary Slam and Telekinetic Overload) in one build.

Loki

His frost magic has been removed as a dedicated tree and integrated into sword magic abilities themed after multiple realms, including "Muspelheim, Asgard, and Hel". [Source: Dev Blog 1]
Is receiving a new Ultimate to replace his Frost Giant form. [Source: Dev Blog 1]
While receiving new sword powers, Loki's staff will still remain the dominant weapon in the majority of his powers. [source]

Luke Cage

Is receiving a new Ultimate. [Source: Dev Blog 1]

Human Torch

Spirit is being exchanged for a new Primary Resource called "Heat". Heat builds up over time as nearby enemies are affected by HT's burns and is not consumed by powers. When Heat is maxed out, HT becomes "Overheated", moves faster, leaves a trail of fire where he moves and can no longer generate Heat. Powers then spend Heat but gain extra effects. He ceases being Overheated when his accumulated Heat is completely spent. [Source: Dev Blog 1]
Fiery Uppercut has become a mid-ranged "Flame Tornado" attack and is given as an example of how Talents can alter powers in Dev Blog 1.

Invisible Woman

No heroes are capable of a "full support" build, but Invisible Woman may have "the most" team support abilities. [source]
"Sue gets more bubble shields, improved Crusher AI, talent to change Crusher entirely, team support, charges instead of Focus". [source (Twitter)]

Mr. Fantastic

"New Signature, Bouncy Ball form remains as non-transform normal power, can stay in bouncy form without losing your power bar!" "You'll drop from ball form when you [use gadgets], but the gadgets will still be on your bar, not a bar full of "Bouncy Smash" buttons". [source (Twitter)

Rogue

Instead of receiving Talents, Rogue is getting a "power library" to hold every powers she steals indefinitely. She still must steal a power before it becomes available, but once she does she will never have to steal it again. This feature may not launch until a month or two after the Talent system goes live. The new system will not know powers stolen before it launched. [sources 1 2 3]

Storm

New power hint: "That awesome cyclone we made for Storm?"
"Storm shall darken the skies once more..." [source (Twitter)

Taskmaster

Has many new powers. [source]