I gotta say, I'm loving what they're pulling into this. The rumor mill said that they were trying to balance out all the things from all the editions that were good.
I'll use the wizard class as an example, as that's the one I'll be playing in my playtest game.
2nd Edition Throwback: AC, while still going up as we've been used to in the last few games, still seems to be more limited in their advancement. Also, levelling up doesn't incur tremendous HP gain, so there's an element of danger to be found. (One nice change: The Wizard started with 16 HP at the beginning, so it takes 2 or 3 good whacks to drop him!)
3rd Edition Throwback: Aaah, we have Hit Dice (Sort of), we have spells that we eventually run out of, and they function JUST LIKE I'M USED TO! Magic Missile is Magic Missile, Burning Hands is Burning Hands, and Sleep is still a save or die effect. And we have Saving Throws, but your throws don't improve as rapidly as they did in 3rd Edition. Of course, most effects have a lower DC early on.
4th Edition Throwback: CANTRIPS! Cantrips are essentially at-will powers, which is one thing they got right in 4th edition. No longer is the wizard the "Cast one and done" guy. He has versatility and STAYING POWER which make sure the guy playing a wizard is still able to contribute even in the fifth fight of the game.
There's probably more, but that's what I know for now.