They adjusted most of the classes so there are few, if any, "dead" levels (Levels where you get nothing beyond HD, BAB, Saves, Skills), and by far the most tweeked class is the ranger, but most of the others were touched.
Generaly, my feeling is that the power level of non-spell casters and non-spellcasting abilities was bumped up a bit (more class abilities, many feats reworked, more feats, skills tweeked, etc.), and most pure spell casters got a bit hosed due to changed in many of the spells (new dice caps, duration reductions, changed spell mechanics, etc).
Overall, I think that it just works a bit better, but it is definately different. Not quite a tue 4Ed, but getting close.