Big changes you'll notice:
1. There's lots of size changing in combat. What with the new versions of trip, the Improved Grapple feat, and new monk flexibility, there's more advantage to large size than ever and a first level spell that can give it to you. Later on, polymorph steals some of its thunder but that's that. With size changing comes squeezing rules for when creatures can't normally fit into an area.
2. There's a lot more necromancy than before. Ray of enfeeblement is now nearly as good as magic missile (IMO). They've added Blindness/Deafness to necromancy and a lot of exhaustion/fatigue effects. If someone in your party (or the DM) decides he likes this, there will be a lot of adjusting stats, encumberance, and attack values on the fly in combat.
3. Blasting things at range is a little harder to do. Haste doesn't give extra actions so you can't toss 2 spells/round without quicken and GMW on bows and arrows no longer stacks so archers aren't quite as nasty as they were in 3.0 As a result of all this, combats tend to last slightly longer than they did in 3.0
4. You WILL need to know the grapple and trip rules because people WILL use them.
5. Non-combat time scales are significantly shorter in 3.5 In 3.0, a lot of buffing was hours/level with some important stuff at ten minutes per level. This made the 30 minutes to 5+ hours time frame the ones adventurers were focussed on. In 3.5, most of the buffs are 1 min/level with some at 10 min/level. This means that, at mid-levels, the time frame adventurers focus on is 5 minutes-2+ hours. Being able to guess fairly precisely when danger may arrive and deal with it quickly is more important than it was in 3.0