I initially wanted to answer "you're crazy!," but it occurs to me that 12th level is usually the height of my characters' careers. They've got a simultaneous level-derived feat and ability-score increase, they have enough experience to define them as characters and enough power to take on cool monsters with neato tricks, but not so much that they mow down all opposition. Twelfth level characters have access to sixth-level spells, but are still reasonably cautious about third-level spells. Fighters get three attacks, while rogues and wizards still get only two, and there's a fair discrepancy between the skill behind those attacks (+12,+7,+2;+9,+4;+6,+1). Likewise, there is a significant differentiation between skill-using and non-skill-using classes (24 pts vs. 96 pts).
I guess I just like 12th level.