The most important thing in Barca that is overlooked is they play exactly same players and formtation either its Real Madrid or Manchester United or Malaga. This helps the team to be clear in their roles and playing exactly the same way they do every 50 games makes them more known to the system and player around.
Their is no team in the world who do this. Therefore as soon as they face Barca, they suddenly have to change something and thus makes them weaker than they actually are against other teams.
For instance, just think there is a match between Barca and Man City tomorrow. Pep and his players will play the exact same player and formation they usually do and so are relaxed and concentrate on the specific part of their game to exploit ManCity's weakness. Whereas Mancini and his players has to change their players and formation and they will instantly have more pressure and little time to prepare. Mancini can't play the exact team he played against Tottenham as it will mean Toure and Barry against Xavi, iniesta and Messi in the midfield. Barca will eat you alive if you play with just 2 midfielders. This forces Mancini to play with another DM and ultimately sacrifice one of Nasri/Silva/Aguero/Dzeko and this suddenly makes you weaker in attack.
This is what makes Barca the best team in the world. We forcefully make other teams weaker making them sacrifice their own game plan. Before the match is even played, we make them weaker than they are against other top teams. But we, we just continue and play and suddenly those other 49 games we played looks like training match for us to be more known to the system, players and formations.
This is the reason we almost always has an upperhand against our opponents and add Messi, Iniesta and Xavi(3 players in top 5) which makes it even sweeter for us fans.;)