Wow… noone ever replied to this post? How rude.

The nutshell answer is; not much. Just a few of the rules got tweaked to better balance things. Some spells got nerfed, some got pumped. For example, Entangle and Hold Person allow ways to get out of the spell, so that it's not a one-hit-killer anymore.

Yeah. The objective wasn't really to alter the game, per se. It was really more to "fix" (for lack of a better word) some of the issues with the d20/DnD 3e system. It factored in all the errata, as well, which is really nice.

Basically the reason people switched from 3.0 to 3.5 is because physically it's the same thing, just with some clarifications. That, and they have this habit of no longer supporting previous versions, so if you wanted new books, you'd have to understand the minor tweaks.

So basically you can see it as 3.0 being like a Mountain Lion, and 3.5 being like a Black Panther. They are basically the same thing, but with minor alterations. And 4.0 is a Polar Bear. It's a mammal..and that's about where the similarities lie.