delericho said:I would suggest instead that you word the limit as "your caster level in any class cannot exceed half of your character level (rounded up)".
This deals neatly with the Mystic Theurge loophole mentioned earlier, but it also means that a character who takes his first four levels in Fighter and then multiclasses to Cleric has the option of gaining his next few levels exclusively in Cleric to "catch up". That may, or may not, be desirable.
Oh, and I also probably wouldn't play in such a game. But that's just a matter of taste, rather than because what you're suggesting is inherently badwrongfun.
I think that's a better way to than the "no consecutive levels" plan, unless you don't mind said loophole.
I'm curious though - is the campaign designed to be very little magic in general (and hence high level magic is extremely rare) or are you hoping to have a lot of low-level stuff, but very little high level? The two may require different rules tweaks to achieve.