D&D 3.5 (and Pathfinder sometimes called 3.75) is a system like the old D&D (archtypes, levels) that is heavily skill and feat (special moves) based. 3.5 focuses on making any character or monster in a well defined way. Basically all special powers are defined so in theory a player could play any monster and IN THEORY you would know how to balance that character with the rest of the party.
D&D 4 seems inspired by video game RPGs like WOW or Kingdom Hearts. Often different characters are equally powered (i.e. do same damage) and only the special effect is different. This is only true of the basic characters. There are some new classes that have other tactical benefits and I think 4.0 is the first to actually think about group tactics and powers in any serious way.
A lot of old times are very anti-4.0. I am not. I don't like it more than 3.5 but I don't think its as evil or lame and a lot of people. Its just different with its own focus. 4.0 does seem more combat oriented (if you can believe that).
Both versions are superior to the older versions of D&D (in my opinion). You get more options and control of what your character can do and focuses on as you go up in levels. Many people would disagree with that. I think magic and thievery were broken in older versions and the newer games improve that. Lots of people would argue which is better and why, which better supports role playing but I don't see that any different than arguing between D&D and other systems. Basically, if you consider what each system tries to do you can get a good idea of what it will work well for and what it wont.
Between these two games (3.5, 4.0) I think most people would agree that 3.5 will appeal to old timers more because it is more like the older versions. 4.0 will appeal more to people who grew up playing RPG video games, including the ones that are multiplayer and cooperative.
The only thing that you can say entirely objectively (I think) is that 4.0 scales combat in a different way than all previous versions. At high levels it is designed to make combat last longer and you have to think more as a group. All the older D&Ds were much more likely to have Bob does X and Sue does Y and combat is over. 4.0 is more focused on combat as a team as that was part of its design.
Now I will watch as the different camps tear everything I said apart.