Pilots who have no concept of the whole distance/speed/altitude concept.
ATC gives a discretionary descent, and he just shrugs and says, "Eh, let's just start down now," even though we're still 150 miles from the airport (bonus "peeve points" if we've been enjoying a tailwind and smooth ride at cruise altitude the whole way)!
He starts a looooooong, slow descent, and somehow, because he is paying absolutely no attention to the whole 3 dimensional picture, still winds up too high and has to use the speedbrakes to get down!
And, for the record, I'm not a "speed brake nazi". If you need 'em, use 'em. Just try to use some sort of descent planning, for crying out loud!
LAXSaabdude.