Well, I came up with a system based on some musings of Monte Cook's over at his website. I haven't done much with it yet, and it's gonna be a big project, but it would address that very concern.
At the most basic level, you have a mana pool that regenerates at the rate of one per round, as long as you don't cast a spell in that round. Simple spells cost a certain amount of mana, and consist of the simple, easy-to-use effects that aren't a big deal if they can be done all day (albeit with a rest period of a few round sin between to let your mana regenerate), such as simple damage, defense, and such things.
Then you have Complex spells, which do the more powerful or bigger effect. These include Cure X spells, summoning spells, Polymorph, and the other big ones, as well as any spell with a casting time of longer than one round (because it makes sense... they take longer to cast because they're... complex spels). The thing is, that any Mana spent on a Complex spell isn't regained until you get your 8 hours of rest.
It comes together with both ideas. You can do the basic stuff all you want, with a few rounds of rest in between. But you still have to manage your resources when using the bigger, more widely effective spells. Not only because you won't be able to use the Complex spells but so many times a day, but also because using Complex spells means you have less Mana left to use your Simple spells, as well. Best of both worlds, as it were.
The main problem is that it'll take a ton of work to get running, for just one person (me, that is). I need to either come up with a whole new list of spells designed for this idea, or go through all the existing spells and divide them into either Simple or Complex.
But if WotC used this system for 4e, I think it would address everybody's concerns on the matter. It would be slightly more complex, but I've actually tried it with a sample spell list, and it's not much more complicated than keeping track of which spells you've prepared and which ones you cast.
