Healing and Hit Points--you only are allowed a certain number of "healing surges" per day, barring powerful magic. So your body can only really survive being patched back together so many times per day. The whole "you fight, I'll heal you constantly" thing doesn't work.
Aaaand, the amount of damage you heal is based on your CURRENT hit points. so, if you're at half your health, a heal will just about patch you up. If you're down to your last few, a healing spell will patch you up a little bit, but you're still going to be hurting.
What's that mean? The top levels of HP might represent scratches, luck, endurance, et cetera, not "Real" damage that's hard to repair. Just like it was always supposed to be :)
Multi-classing--it doesn't seem possible to multiclass in the 3.5ed sense of the word--you can't play a Cleric/Ranger/Thief. However, you can use multiclass feats to swap in elements from another class, and create what's effectively a hybrid fighter-mage or whatever. Interestingly, one of the feats that's not too difficult to swap in is the ability to learn and cast Ritual Magic, so making a mystical thief should be pretty darn easy :)
Bards--bards are gone. Warmaster is the new bard. It's a bard with big stomping boots and a spiked lute that beats people over the head and inspires the party whilst doing so. Mothers, hide your sons. This is an opinion, anyway--warmaster is the big "buff your friends" class now, but a lot more active and dangerous than the bard was, because most of his inspiration effects require actually being in battle.