Looks/style/intrigue. Has to be interesting or something I don't have. Otherwise I have a bike I can already ride.
Fit, although I have found anything from a 54 to a 62 ctc(seat tube) I can fit. Found I just need taller and longer stems+longer seat posts on the small side.
Quality frame+componentry. I mostly do this so I don't end up with a million bikes.
Lives rent free in my brain for a couple days. Found this to be a good test on whether I really want a bike. If I wake up the next day or two still thinking about it.... then that's the sign.
I am okay with using my parts bins to get bikes running, or bad paintwork since I just treat that as a way to improve my painting.
Deal breaker would be cost so many deals are out there I am okay with waiting for something interesting to show up at an okay price.