Perhaps the biggest stylistic difference you'll notice is that while the use of a grid and miniatures is presented as a very good, but optional idea in 3.0, it's integrated much more into the 3.5 rules as a default method of playing. You can still play without minis, of course, but all of the examples of play in the PHB will frame things in terms like "squares moved" rather than "inches moved", and advises you to place the center of any area-effect at a "grid intersection".
Depending on your attitude toward minis and battlemats, this may make you feel like the game has shifted more toward a wargame than an RPG. For myself, I think it just does better at explaining how combat maneuvering works using tools that I was already using.
Several spells have been decreased in power from 3.0. Harm now does dice of damage rather than simply taking its target to only d4 hp remaining (avoiding the Harm/Quickened ILW combos). Haste doesn't give an extra action, but an extra attack in a full attack action, making it a better fighter buff that a caster buff. The 'animal buff' spells (Bull's Strength, etc) had been reduced to durations of 1 minute/level, making them useful for only a single encounter rather than an entire day, but they now give a flat +4 bonus rather than 1d4+1.
Some classes have changed. The ranger's abilities are now spread out throughout it's levels, rather than being front-loaded in the first two levels, and there is now a archery-based option for them. Paladin's mounts are now summonable creatures (a trait commonly referred to here as the "PokeMount"), sorcerers and bards have the option to change out spells as they go up levels, and the monk has been reworked to be more in line with the other classes (they no longer have a separate BAB progression for unarmed attacks).
Damage resistance changed. Now you must have the specific material named to bypass DR, rather that having that 'or better'. For example, in 3.0 you can bypass a lycanthrope's DR with a silver weapon or a magic weapon, which was considered 'better' than silver. Now you have to have a silver weapon, even if you're using a +5 sword. Also, DR that is defeated by a magical weapon is no longer rated by the 'plus' of the weapon; you just need a magic weapon, whether it's a +1 axe or a +5 holy avenger. Some may complain that it leads to a 'golf bag syndrome' where PCs have to carry around multiple weapons with different properties. I haven't found this to be the case, as in general they also decreased the amount of DR creatures get (so something that was DR 10/silver in 3.0 might be DR 5/silver in 3.5), which means you can still kill them without the special weapon, it just takes longer.
Overall I approve of the changes, though some aren't so great IMO. Part of the defining everything in terms of map grids caused them to define all creatures as occupying a square area. Thus horses take up a 10x10 space, rather than a 10x5. While I understand the logic, it still feels wrong. They also now give sizes to weapons that are comparable to creature sizes, and you take a penalty if you are using the wrong-size weapon. Thus a halfling is fine using a Small shortsword, but not a Medium shortsword. My main beef with this is that as far as I can tell there was nothing wrong with the way weapons were defined in 3.0, and I don't see the need for the change. With the other changes, I can at least see why it was done.
Honestly, there's not enough difference between 3.0 and 3.5 to make the older version any less fun. I'd suggest getting a copy of the 3.5 SRD online and looking through it to see what you like and what you don't, and grabbing the stuff you like.
