During the scion class discussion, there was some talk about the problems with spellcasters balancing with other classes. I suggest some things you can do to bring powerful spellcasters a bit down to earth without outright prohibitions.

Traditional casting from memory could be limited to lower level spells, and higher level spells can be restricted to spell storage.

Second, where powerful spells can be cast can be limited. I employ two kinds of places. All examples will be priest class oriented, because this is where I have done my work. I'll offer some wizard suggestions, but haven't used them with PC's. Mid level spells can be cast in a place that has been sanctified. A druid might pick an appropriate glade and sanctify the place, allowing him to cast mid-level spells in this place. Mostly he would just create some spell storage devices, but it does allow for the refuge of a party to be especially well defended if the druid has access to more powerful spells from memory. Each priesthood would have its own right places and times. High level spells would be reserved for permanently holy places. So resurrection could only be performed at an established holy site, not on the fly or at a hastily sanctified refuge. Likewise preparing a Storm of Vengence scroll could only be done at an established holy site. For wizards, you might say that medium level spellcasting requires so many books, references, componants, and special preperations that it can only be done where I can reasonably control my enviroment. Pulling this stuff out on a street corner, wooded trail, or dungeon passage exposes all my spellcasting paraphenalia to disruption, capture, or breakage. Further, high level spells might require that I be back at my own permenantly established labratory. So, if I can rent a room at an inn, and be left alone to set everything up, I can cast a Remove Curse, but to cast Wish, I need my lab, or one that is super-well equipt (my mentor, another wizard with four more levels, the Academy in Anuire, &c).

As a practical matter, what this does is say that the more powerful a spell is, the more advanced perperation I need. D&D has always required that I plan out tomorrows spells, rest some time in preperation, and collect componants. This just extends the preperations required so that medium and powerful spells are only practical as scrolls, wands, or in other storage devices, ie well prepared.

Classes that use mundane powers of sword swinging or sneaking or persuasion are more flexible in that I can attempt to hit, sneak past, or persuade all the time in an almost unlimited capacity. Classes that have powers that are substantially more powerful, like high level spells, are much less flexible. The game already assumes that, employing a spells per day chart, and no such limits to how often I can attempt to bonk a goblin with a mace. I just carry that trend further.

One of the nice features of this approach is that the DM can balance it very much to taste by being more or less strict depending on how much they feel powerful spellcasting needs this effect.

If one wants to add further complications, one could require cooperative casting for the casting of high level spells (or even mid-level spells by characters who have only just mastered them). This creates a nice incentive to have apprentices, too.

Not only does this have a nice effect of creating enterages (which seems to fit the BR flaver nicely), but also can be scaled up to the realm spell level. I need so much help to cast Bless Land that I actually need holdings to get all the cooperative casting and assistance neccesary to pull it off.

salt to taste
Kenneth Gauck