I think for me, an interest check has to be, first and foremost, digestible. Keep it short and simple. Set up the base plot, what kind of story it'll be, what sort of posting schedule will be expected, and anything else important, but keep it short and simple.

Also, don't fall into the trap of writing the interest check as it if were "in-universe". Keep an outside perspective talk about your RP, don't show me a slice of the world you want people to play in and have them dig through that to find whatever it is you want them to find.

"This Rp will be in a fantasy world where people have fish for hands. Figure that's a fun setting. Will have some more info in the OOC." Not "GHeihewhfh, has a deep history, and within it, countless wars have been fought between the countless races, in the end, the FHOOHFEH won, killing all other..." Or, at very least, if you have the "in-universe" stuff, do not put it in focus, and do not expect people to read it, so if it is essential for your rp, re-post it in the OOC.

Finally, I think what truly interests me in an interest check, is the DM. If the DM seems like a fun person to play with, I might be very interested in joining an RP I'd normally skip over, while if the DM seems a pain to work with, I'll skip the RP, even if it's the best damn idea I ever saw.