I think one thing that has caused me to drop recent roleplays is the feeling that I will be unable to achieve what I want out of my character. A lot of the times, I overpredict the arcs I want them to go through and then can't compromise with the limitations of that game and unpredicted nature of RP in-general. That was Tohru Anayo, my first 'Past of the Past' character in a nutshell.
Another thing that can kill the fun of a character is banking your enjoyment on their reception in-character/their "role" in the cast. Since there is no central protagonist in any group roleplay, and because everything is dependent on in-the-moment writing and player interpretation, it's impossible to predict how people, and by extension their characters, might interpret your own. And it's equally difficult to tell how a character will develop because of those factors. I went into 'Cornered' expecting Rina the Chimecho to be this sychopantic, point-obsessed mastermind, but I blew it the moment the Game Master didn't give her (myself) immediate credit for doing an obvious good thing. So she had no facäde and just turned into this bitchy, whiny, angsty teenager. Even then, though, it wasn't hard to write for her, and a lot of people in the roleplay actually liked her! I just could never get over how cool she could've been. But characters of that sort of charismatic, "mastermind" archetype are almost impossible for anyone who isn't already on a higher pedestal (the Game Master, basically) to pull off while still being effective because characters and their players are so interconnected.
Both of those examples are from in-character experiences because I think it's hard, for me at least, to tell how much I'll enjoy a character until I start writing them. The best tip I can give is going with your gut -- sign up with the concept that speaks to you the most -- and be flexible from there.
(Or write first, fill out sign-up later. That probably helps a lot, but I'm too lazy to really do it, lol)