For me I'd join a team if the project is good and I'd get a chance to showcase my skills and creative concepts. But the people have to be nice to work with as well, that's critical! Also the project has to be conducted in a 'professional' manner. That is to say, that everyone pulls their weight (me included!) even if it's just a passion project.
Another factor is good 'planning' and specific direction when needed. I think the biggest drain on any artist is 'endless' revisions after they were asked to do something specific beforehand while the director, or whoever, keeps changing their mind or flip flopping on the previously agreed concept.
This is why I send over a lot of rough sketches beforehand and then get approval before the actual piece begins. Personally, I don't mind common sense changes - these happen. But when a client begins to change their mind constantly, as an artist you need to start charging for the extra time you spend correcting the same image over and over again.
Oh, and never use language like, "just make it pop". Because we don't know what that actually means. Just be as clear as possible. :)
Just my 2 cents.