Fighters are now harder and they deal more damage, without being any less straightforward or easy to play than before: they just gain some straight bonuses.
Barbarians are awesome. The rage powers give you the exact impression of a freakin' ragin' force of nature (be sure of getting the update; rage points are not bad, but they are way worse than the -let's hope- final version).
Sorcerers change a lot with the bloodlines feature, and acquire their own niche and personality, instead of being "the lazy man's wizard".
Druids have been hit with the nerfbat. Wildshape is no longer the deal breaker it was in 3.5, although they gain in versatility with improved animal companions and the option of becoming elementalist druids.
Paladins and Clerics gained something to do with their turnings / lay on hands. Paladins become better healers (and gain a mega-cool holy weapon feature if they give up on their mount), and clerics have their channel positive/negative energy (or "healing explosions", as my group calls them).
Wizards gain in versatility, mainly at the first levels, with their now at-will cantrips (good-bye, crossbow!), although their most powerful spells (SoD's, Polymorph) have been toned down.
Monks benefit the most from the CMB mechanic, being authentic trip (and bull rush, and disarm, an sunder, and...) monkeys. They are a great striker/controller: not the one who kills the monsters, but the one who puts them in a plate for the fighter/rogue/barbarian/paladin to finish off.
Rogues gain a few tricks (sort of half-feats at every previously "empty" level), but their main gain has been the ability to sneak attack almost everything.
Rangers are more or less as they were earlier. They have a few more things to do with their fighting styles and their companions have improved a bit, but they haven't changed much.
Bards look cooler, but I can't tell you much about them, because we haven't tested them.
