* Clerics as walking first aid kits with no individuality.
You know, the more I try and think about ways to solve this without just handing out magical healing items or automatically healing people between fights, the more I realize why they created class 'roles' in 4e so that there wasn't a question of what each person was supposed to be doing. It is just easier to deal with, even if it is silly/unrealistic.
Anyway, I had another idea, hopefully this one will work better than the others. You stated that you want the characters to have to worry about attrition, but that doesn't work if they have an excessive amount of healing items. Perhaps there should be a limit on how much a single character can be magically healed in any given day? That would mean that people can't just throw their hit points around like they mean nothing, because there's a limit on how many 'magic bandaids' they can throw around.
You could fine tune things even more by having it so that there's an even lower limit on how much healing they can get from magical items. A cleric who is skilled at healing should be able to do a better job than a belt of healing.
Say, for example, you allow characters to completely heal up to their full hit points in one day. However unskilled healers using potions and magical items can only heal up to half that limit, and a cleric would be needed to get to the full limit. This way, you've taken some of the burden off of the cleric without letting the party completely rely on magical items.
Or another idea is that characters could go past their healing limits in an emergency if they were fighting something really nasty, but would suffer side effect/penalties later on after the fight. That means they could overheal if they really really needed it, but it isn't something that they would want to do and would try to find ways to avoid it.
Though, these are just some random thoughts....all this might not work and it might add too much complexity to a game.