There are a few notable differences between 3.5 and 4. First, it's less complex. A lot of that was streamlining of rules in a way that lost very little depth, but there are a few places where they did sacrifice some depth, mostly for accessibility purposes.

I'm friends with a lot of die-hard 3.5 edition fans. For the most part, they see the streamlining and view it all as a loss of depth, even though only a few portions of the game actually lost much depth. In a few cases, such as the simplification of the skills list and the slightly less flexible character classes, they do have valid complaints, but for the most part I see it as resistance to change.

I know many people who complain bitterly about the lack of content compared to 3.5 - and by content I mean the sheer number of classes, skills, spells, items, etc. Which is a valid reason to stay with 3.5 for now, but not a valid reason to criticize the 4th edition system as a whole - simply because it's only a couple years old. When 3.0 was first released, it lacked content too, and many of the people who voice these complaints have used nothing but 3.5 their entire D&D career.


4th edition has some really nice improvements, though. First, it is vastly more accessible. Part of that is through presentation and explanation of the rules, but a large part of it is the aforementioned streamlining. The system is simply easier to understand than older editions.

Another improvement - in my opinion - is the way that they've spread out the fun stuff to more classes. I have a friend who only ever played wizards in 3.5. When we switched to 4.0 to try it out, he was veyr angry because he felt like he had fewer exciting, utility, or awesome spells. Which is partially true, but most of those spells and abilities still exist... they were just redistributed to other classes, meaning that the OTHER players actually had cool abilities beyond standing there and swinging a weapon. It was a loss for the wizard players, but a net gain.

Another major difference is that positioning has gotten a lot more importance. A lot of abilities now do things like pushing enemies around, etc. From what I recall, that was less prevalent in 3.5.