There are many factors which I explore before joining an RP. Most of the time it's Plot, story, and the way the RP works, since some RPs can have complicated methods of RPing. For example, even though it's not too complicated, I do tend to stay away from RPs which involve levels, since the process of levelling up can be quite complicated, plus it would mean that the entire RP depends greatly upon the GM for raising levels and such. Some RPs can get even more complicated, so I'll only join one of those if it has a really interesting storyline. In terms of story and plot, it usually isn't something involving trainers, just because that isn't really my kind of thing. I prefer all-Pokemon RPs such as Mystery Dungeon, because there's so much variety you can choose from for your character. I mean, you're a magical creature. Awesome.
While these are the most important factors I take into consideration when looking at an RP, I usually also spare a glance at the GM, and ask myself questions such as "Are they new to RPing or experienced." Or, "Did they write the initial post of the RP in quality." These questions can shed light on both the GM themselves and their RP; often, I find the quality of an RP is very dependant on the skill of the GM.
If you have any specific questions, feel free to ask. :)