Don't know why everyone thinks Real got the tactics spot on, they were certainly not bad though as per usual Alonso got away with studding Alves in the ankle and several other tackles, though Alves got away with, a rugby tackle on Ronaldo and possibly a stud/stamp thing, hard to tell from the one replay how intentional it was).
Overall Barca's problem was the same as many games recently, the "Rooney" effect, Messi was played as the striker, but wanted the ball and wanted it 15 yards deeper than the defence.
Literally the second they put someone on to play as striker, he made a run at the cb's, which the cb's matched and created a gap for Messi to run into. They only managed to do that a few times as their then brilliant tactic seemed to be, "we've just worked out how to create space infront of the defence...... so now lets start with the long ball" .
They did it against Chelsea too often as well, Barca at their best, think back to smashing 5 past Real Madrid, and you had 2 or three players running along the back four, then darting in behind as Xavi/Iniesta/Messi(from out wide) passed on the diagonal in behind. That was the strongest part of their game for years basically, today and against Chelsea you had Messi standing WAY deeper than the CB's, no striker pushing the cb's, Tello/Alves basically as the wingers both with no targets as no one ran at the back four, so couldn't cross nor pass in behind anyone.
All season they've lacked that same run in behind mentality and its started to effect their passing as they aren't looking for that pass as often.
Barca's daft tactics threw that game away, Real, or Ronaldo in particular certainly did brilliantly and all they could but, considering how much posession they had again and how in the only really truly effective minute or two they played all match they scored... had they played like that all game I think they'd have won comfortably.
Messi doesn't like playing as a striker, shouldn't play as a striker and desperately needs a striker to help create space for him to run into. He didn't look happy today or against Chelsea, he's being asked to, for no reason, do something that limits his game far too much. It also means he's basically the only threat and can be marked fairly easily.
The last couple games as well with Alves basically on the right, Messi as a very withdrawn striker, no actual striker and 3 central midfielders, they've had too many central mid's sitting in the same area getting in each others way.
Defensively, pretty much wrong line up, wrong tactics, wrong substitutions(Xavi shouldn't have gone off, nor should Tello, or Adriano) and FAR too late in the game.