I was going to make a post on this at some time, but Blindsight on the official forums made a much more comprehensive guide than I ever would.
I was surprised it failed to mention that many non-RPers tend to be attracted to RP servers simply because the average age of the population tends to be older (and therefore somewhat more mature). Most of the other points are quite insightful however.
As with most choices, you will never find the perfect server. Your best bet is to make a list of the things that are most important to you (in priority order) and pick the server that meets the most of your criteria.
Or… you can pick a name which sounds pretty to you. But I don’t know anyone who would ever do that.