Information based on posts by Ghostcrawler and the official preview. This list is, of course, not complete.

Death Knight
This system will activate with the first talent point received. (Source)

Mage
Frost - Summon Water Elemental

Paladin
Retribution - Divine Storm (Source)

Priest
Discipline - Penance
Shadow - Mind Flay (Source)

Rogue
Assassination - Mutilate
Subtlety - Shadowstep

Shaman
Elemental - Thunderstorm
Enhancement - Dual Wield, Lava Lash
Restoration - Earth Shield

Warrior
Arms - Anger Management, Mortal Strike
Fury - Bloodthirst
Protection - Shield Slam, Vitality (Source)
I would not focus too much on the old masteries, except for the third one that scales with gear. That one is still around.
Vengeance for tanks is still around and dps get damage and healers get healing and that sort of thing but the system has changed quite a bit overall (and partially as a result of community feedback). (Source)
Next beta build will be interesting
We want to unveil the entire enchilada in the next beta build or so. Everyone here worked really hard to get the whole package to a relatively playable and polished state. In essence we're not announcing a future plan -- we're telling you how the game works today. It's all done.
It's hard to offer timelines for when we will do a new beta push, but we're talking days or a few weeks, not months. All of the passives will be visible then. (Source)
Talent Points gain rate
About one every other level, but there are some exceptions. We are trying to alternate new abilities and talent points where possible. You will have 41 talent points at 85. (Source)
Spec abilities scaling
Specialization-defining abilities that are unlocked (e.g. Mutilate, Penance, Water Elemental and Mortal Strike) will scale based on a character's level and reflect an appropriate amount of damage, healing, and mitigation, etc. With the proper balancing, having these abilities at level 10 won't trivialize the leveling experience, but instead enhance it by allowing players to incorporate unique spells and skills that highlight their specialization into their rotations at a much earlier level.
We'll be closely monitoring how these new talent changes affect gameplay in the beta and will make adjustments to the system if necessary to ensure that our original goals are met. (Source)
Dual Spec
Dual Spec won't change. If you choose Protection paladin at level 10, you can respec to Ret paladin at your class trainer. Once you unlock Dual Spec, you can have 2 Prot builds or a Prot and Ret build or whatever you'd like. It will work almost exactly the way it does now. (Source)
New Talent system is ZOMG AMAZING
It's ZOMG AMAZING. We wouldn't go through all the work it is taking if we weren't convinced it was 100% win.
On your first point, too many of the specs didn't really feel like their spec until they had spent a lot of talents in the tree. All mages pretty much played the same until level 30 to 40 or so. Even with the Cataclysm changes, a Holy, Prot and Ret paladin would pretty much be using Crusader Strike and Judgements until they had several more abilities unlocked.
On your second point, we know some players liked the option of spending some points in the first tree and then some in the second tree. Often this was just a trap for new players though because it delays your getting the 21, 31 and other "gold medal" talents that really define your tree. Experienced players might learn to get a 5/5/5 build or whatever and then respec once they could reach the 31, but we could also just make better trees that didn't ask you to do that. (Source)
How big are the new trees?
They have 38-42 or so points in them. You could spend all 41 points in one tree, though often you'll want a 5/5/31 or a 0/7/34 build instead. Every talent is 1, 2 or 3 talent points. Earlier levels have about 8 points on them (typically a 2, 2, 3) and deeper levels have about 5 points on them (typically a 1, 2, 3). (Source)
New Talent trees overview
They look superficially like vanilla talent trees, but they are far more interesting.
Almost without exception, the talents we cut were the ones everyone took or the ones nobody took. There will still be very attractive talents that probably everyone takes, but there are also definitely choices where you can only take A or B and want both. Typically you might see a useful (but not necessarily dps-enhancing) ability in your own tree, but also a really attractive one in another tree.
This is a good thing. It won't surprise me to see a knee jerk reaction from some players that we're removing their favorite talents and "dumbing down the trees." However, I'd challenge them that there can't be that many players out there that play Arcane because of Mind Mastery or Arms warrior because of Strength of Arms. Yet, if you skip either of those talents, you're just making your character less effective.
A Cataclysm goal is to remove lame or boring talents. Yet we don't want to replace all of those with procs or game-changers. Really the only solution is to have smaller trees where nearly every remaining talent is a big deal. (Source)
Hybrid builds and active talents
You typically won't get active abilities from a second tree. The most you can get in a second tree is 10 points, and usually the active abilities are deeper than this. We think it makes the distinction between say a Frost and Unholy DK more pronounced when all of their talented active abilities are different rather than being able to cherry pick the best from each tree. It does end up killing any chance for a build that goes halfway down two trees, but usually we would consider such builds failures because it meant the bottom of "your" tree wasn't good enough. These were most popular in PvP when players felt compelled to get "must have" PvP survival talents in multiple trees, and those are the kind of talents we want dead anyway.
With these changes though, the talent trees are going to be even more rebuilt than we were already planning with Cataclysm. Talents are moving all over the trees and every tree has new talents as well. We'll give you a chance to see the trees soon (tm). (Source)
Current talents are too complicated
The new model theoretically buys us several more expansions of not having to mess with the overarching talent tree design. In a broader sense though, one of the challenges of working on World of Warcraft is adding new content without making the game totally inaccessible to new players.
I have used this example several times, but we have one very senior designer at Blizzard who isn't working on WoW but obviously knows the game very well and has been very active in PvP and raiding at various points in time. He took a short break from the game (I suspect to play Modern Warfare 2) and when he came back, we had messed around with his class's talent trees and reset his talent points, so he felt like he couldn't jump back into the game again without spending 45 minutes researching what the community thought a good build would be. We're hoping with simpler (but still deep!) talent trees, players like him will feel more comfortable picking their own talents. He might eventually go poke around in theorycrafting forums to min / max his build, but he won't have the moment of being completely overwhelmed when looking at a blank talent tree. (Source)
In previous expansions, more choices were given to the players in terms of how they built their characters. Players were freely given the choice to spend points however they want.
[...] While that is true, they were also given ample opportunities to make mistakes, what we call "traps." A forum-savy player may know which are the dumb talents nobody takes or which are the mandatory ones that might at first glance seem too bland to take. But why have "choices" that are just there for new players or people who just want to swim against the stream just to be different? We'd rather have actual legitimate choices, which we feel like we can offer by having a stable of fewer good talents.
To use one of the infamous GC analogies, the best restaurants are not necessarily those with a lot of entrees on the menu. The best restaurants probably have very few choices, but you know that anything you order will be great. They don't bother cooking it if they know it will suck. That's a real choice as opposed to the diner with fish on the menu that everyone, ever the waitress, whispers under their breath that you should never, ever order. (Source)
Dungeon Finder and new talent system
We expect a lot of players to use Dungeon Finder while leveling up. The feature works great for that, and we're spending some effort to update the older dungeons to not be so overspawned and in some cases to even fit into the zone quest content better.
Likewise, we're making an effort to let tank and healing specs be more effective when soloing. DPS specs will still probably be more efficient leveling, but if you just love the Resto shaman tree, you should be able to quest and run some dungeons without even having a dual-spec. It won't be spectacular, but it will be a huge improvement from today.
Tanks and healers will get basic abilities before level 15 (when Dungeon Finder is available) to do their jobs, but it's also true that the lower level content is more forgiving of your talent spec. (Source)
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