OU definitely lacks diversity, balance, and most of all uniqueness. One reason I stay (when I do battle) is because I put a fair amount of work into my team and I'm too lazy to build another suited for other tiers -- despite the fact that I've heard nothing but great things about them.
That, and OU is kind of my comfort zone. For a while it was all I knew, and I'm sure that level of predictability makes up a fraction of why I keep ending up back there. ._.;