I don't think Taylor is that bad individually, but he doesn't seem to breed confidence in the defenders and goalkeeper around him. He's a defender of 'extremes' - he's incredibly gung-ho and 'the charger', if you like, which isn't always a good thing.
I think Williamson is a more level-headed version of Taylor - physically a bit weaker, a tiny bit slower, but his reading of the game and knowledge of when to take one for the team by getting a card and when to simply hoof the ball clear is very valuable.
Short version is: Williamson compliments Coloccini better than Taylor and therefore should be in the first choice pairing.