I normally see who the GM is and check out other roleplays they may have been involved in GMing. I have to admit I'm a little (or a lottle ^^;) spoiled and picky after being GM'd by the lovely [@Dervish]. If the person isn't someone I know, or I've been told to be a little wary, I'll probably scrutinize the details like an evil stepmother looking through Cinderalla's list of chores for a spec of something that was left unattended. 

I like to see a little plot and premise even in the interest check so that I know the GM has a plan of what they want for the roleplay rather than just scoping for people to join when they only have an inkling of an idea. At the same time something that is just a lore dump will turn me off. I want to be interested and not feel like I'm reading up something for a history exam.

I don't think I've ever been turned off by an OOC after being enthusiastic about the Interest check. What is most important to me is the GM's dedication and enthusiasm, which you can normally gauge by how active they are, how they communicate with those interested, and the time and effort they've put in their work.