Pretty much agree with what everyone says regarding commissions, respect, and other issues.
Another pet peeve of mine are cosplayers who are careless with their props. I know it's fun to pose with your prop and all that, but don't do it in a crowded hallway and be considerate where you put it when you sit in a panel or video room. Almost got hit on the head with Sailor Pluto's time key which was made from a metal shower curtain rod.
Cosplayers are obviously the most visible part of a con, especially to regular people. If you do something rude and obnoxious, regular people (including parents of younger cosplayers) will see all of otakudom in a negative way. Sometimes cosplayers forget that or have the "let's freak out the mundanes" mind set.
2011 costume plans: Suzaku (Yami no Matsuei), Lyanna Stark (A Song of Ice and Fire/Game of Thrones), Rowena Ravenclaw (Harry Potter)