just about anything,
look for straight stays, anything that has "sexy" curves to get close to the wheel is going to interfere with the chain, wear through and break.
disc brakes on the rear can be annoying to get around, what with sticking a sprocket in there.
try to avoid alloy frames...hi tensile or cromo is best.
24" wheels "usually" have 32 spokes and prefer 8 hole sprockets rather than the standard 9 holes for 36 spokes. so go for standard 26" rims.
fat downtubes can be annoying, requiring special mounts. try to get something with standard skinny 1" tubing.
rear suspension frames usually dont work. avoid. the few that do are hard to find. read, impossible to find...
there can be issues with the front derailleur interfering with the rear mount. remove. you wont need it. same as drink bottle lugs.
make sure the frame is "deep" enough. usually 16" or more. anything smaller can be a tight squeeze, not necessarily impossible, just awkward.
the scrapyards are far better sources of GOOD frames. amazing what people throw out... Kmart is a no no!
and dont waste your money at the bike shops... or tell them what youre doing!