When an RPer contacts me with interest in an RP I wish to start and states they wish to be a Co-GM when no such position was offered in the OP. Then, I look at their history, to see what they're like, and notice they have done the same thing with every other RP they showed interest in. 

To me, this says they want control/authority over just playing the game. For me, I just wish to play a game, but don't care for the way most others run it, so I make one myself. So, to see someone striving everywhere to be a Co-GM is a big red-flag for me. I don't know why, it's just irksome. 

Another thing is when I see an RPer post a CS, then follow the feedback by the GM (which is a combination of critique, gratitude, and questions) in a very positive manner, and then the RPer throws a fit on how unfair/biased/etc. their criticisms were. I mean, you're going into their RP where they have clear-cut rules, even if they're still being developed. To be so close-minded and unwilling to compromise with the GM in a mature and level way is as fascinating as it is irritating.