Second this, unless it's for a scheduled layover or something else where you're reasonably certain that you won't need to talk to an agent. If it's a weather or mechanical delay, to me this factor would trump all the others.

If it's a long scheduled layover, depending on the locale, the weather, and any possible visa issues, consider leaving the airport instead of sitting in an airline lounge. Public transport to the nearby city, two drinks at a bar, and an hour of wi-fi if you can't find it for free, will probably cost less than a lounge day pass. Immigration folks at popular connecting points like London are quite used to people with long connections doing this. There are lots of threads along the lines of "Is AAA hours enough time to leave BBB?" or "What to do in XXX for Y hours?" in the appropriate Travel forums. If you don't find one for your layover point, start one.