Before I get into the lv 80 stuff, let me just say that this new talent system will make leveling a rogue alt (and possibly other, lesser classes, but why would you want to level other classes when you can level a rogue, right?) exponentially more fun.
Instead of having to sinister strike spam your way to level 50 before daggers actually becomes viable because of Mutilate, choosing Assassination at level 10 will give you Mutilate, and 20 more energy. Obviously you can still choose Combat, which will give you Blade Flurry (though not the one currently sitting in your Live realm talent trees), or you could go Subtlety and get Shadowstep.
Ideally you have a pair of heirloom daggers around, because if I remember correctly (good) daggers are pretty hard to come by, but perhaps Blizzard will revamp the old dungeons some more and put a few dagger toys in there for us.
In any case the low level landscape will drasticly change, both PvE and PvP wise.
Interesting to note is that, for example the +20 energy bonus will also apply to lv 85 raiding rogues. Vigor and Improved Vigor were those things you never had the talent points and glyph slots for, but would still be nice to have. If you will be raiding as Assassination in Cataclysm, we'll be doing it with a bigger energy pool, which might come in handy for energy pooling.
On an semi-related note, I wonder how fast my guild will level up and have access to guild perks. We're a fairly big and active guild (usually hovering around 100 accounts) so the first wave of Cataclysm leveling should probably level the guild nicely, between questing and instancing together.
Rogue vs Discipline Priest
23 hours ago