The major difference I see so far is really an issue of quality. 3.0's release saw alot of flawed books pumped out, riddled with errors, and even the designers hemming and hawing about eratta of certain "broken" things. The splatbooks of course were probably the worst offenders, but plenty of the hardbacks were pretty bad too, Deities & Demigods, MM2, BOVD, & the Epic Level handbook are ones that immediately come to mind for me. Plenty of stinker adventures and WTF products like the HBG/SHBG too. Despite all the disappointments, there were some real gems like the
3E Manual of the Planes and the
3E FRCG though.
Having recently bought several of the
4E releases (after owning only the corebooks since last year), the quality I feel is to a higher standard-ruleswise and utility-wise. It seems the
4E designers have a far better handle on the rules system in less than a year than the
3E crew did over a couple, and they have a clear direction on the business model as well (whether you like said direction or not). Not to mention the eratta has been far more sparse. I don't know if the quality will be sustainable over many years, but so far, I've gotta say (*gasp*) I'm far more impressed this time around with
WOTC.