Except one individual ability in particulare, the physical decline of so many of their players. You did mention Xavi, but also Iniesta, Alves, Messi, Mascherano, even Busquets and Pique I would argue, have lost speed and quickness, and this has affected not only their ability to press, but more importantly, their ability to dribble past opponents.
Tiki taka worked not only because Barca players were good passers and decision makers, but because a critical mass of them could dribble past one, two, or even three players, especially when aggressively pressed. Today, Barcas passing is as good as ever, but now even Messi sometimes gets dispossessed one on one. Unheard of. This makes it easier for the opposition to both selectively press and to park the bus.
This, in my opinion, is the main reason why Barca need variation because they just dont have the ability to walk the ball into the goal any more when faced with really strong opponents.
