Probably listed already, but I really get frustrated when a potential partner has no direction for an idea and has no interest in collaborating on fleshing it out. Basically just: "here's an idea, develop it for me". Sucks the fun out of writing. 

Might be out of character for the post, BUT one of the anti pet-peeves I have is when a partner tells you something like: "Wait, your character was holding that in the whole time?! I never saw that coming!" or "Come on, really, we're going to kill him off? We need to talk about this." Having a partner that's involved in the process is just...amazing. 

There, something positive to end on.