The thing about skill being the most important thing to look for while may be true, is not feasible really. I mean, I hate pugging for that reason precisely. Because I have no real way of knowing if the other people suck or not.
With guildiees, I have a pretty good idea of who is good or at least how they play. I think one of the most important things is group-synergy really. If you know how the people you run with move and handle different situations, then you're like a well-oiled machine in dungeons.
For example, I know I can count on shawn cloaking and ressing me when I get downed, just like satomz's boyfriend portals me through places because I'm a clumsy buffoon. I know I can count on kokabel setting things on fire going "woooooooooooo" or Ella (a guildiee) aggroing everything and their mother =D
That sort of thing you know? just knowing your people.