I'd say too tall is when you find it a pain in the *** to do maintenance. Also take into consideration how tall your stand will be. You'll want your stand to keep your tank at eye level, so you don't have to hunch down. If your stand is 42" and your tank is 30" tall, you'll have to reach 72" high to get into the tank. And also, consider the lighting you'll use. You might have limited clearance from the lights depending on how high they hang (depending on your tank depth, you might have to hang them lower to increase PAR).
Also, you'll have to play your aquascape well. I've seen a lot of tanks with really low live rock and a ton of space on top. It makes it look disproportionate.
I think 24" is the ideal height. It's not too low, it's not too high, and it's not too huge of a pain for maintenance.