Messi's goal seemed to catch the defense completely flat footed. It's as if they all thought "no way he shoots"...and then he did.
2nd and 3rd goals were partly defensive errors, but Barca is unbelievable at creating space and pulling players out of position that in some sense it's not really error just 'better'. One thing I always notice watching MUFC is how good they are defensively around the edge of the penalty area in cutting down angles and preventing the one-two or reverse pass - remember watching games against Arsenal and Chelsea where it looked like those teams were throwing pebbles at a castle when they attacked. Barca ripped them apart around the penalty area, especially in the second half, and that's not really because MU played badly at all, they were just inferior. Messi was great of course, but Xavi is equally important IMO - he might be the greatest ever at understanding and creating space with his passing, just a joy to watch.