WebmasterWorld has a nice thread going about 25 signals of crappy sites. Fribble starts the thread with the notion that we spend a lot of time looking for signals of quality. But it’s also useful to look at the inverse. It’s a pretty good list, and forum members chip in a few other good ideas.
Being aware of these signals can be useful when you are evaluating the merits of web sites for partnering with them. If a site has too may of these negative signals, it may not be worth the trouble to pursue them. While inbound links from third party sites, for the most part (yes I am hedging on this a bit), can’t hurt you, you don’t want to spend your link building energy on things that won’t help you.
After all your time is limited. Of course, look for more than 1 of these signals to tag a site for ignoring, unless they do one of these things in a really egregious manner.