Well, that's the strange thing about the whole Mascherano-transfer. Mascherano perfectly knew what to expect (Pep favoring Busquets over Toure; which was a correct decision of Pep); so he knew that he'd get a lot less playing-time at Barca than at any other topclub. So I'm not sure what attracted him:Honestly, I think Mascherano could be a starter in every club in the world, maybe except just Barca, because of some specifics. For now he's saying in his interviews that all of these rotations are not problem for him and so, but come on - we all know that it's a complete bullshit. It is a problem and soon or later it'll become an official problem. World class players can't be rotated, it's a law.
1) His wife wanting to live in a Spanish-speaking country
2) Having his good friend Messi as his teammate
3) Improving his play (he was already excellent defensively, and now he wants to improve offensively)
4) His fascination for Barca
Difficult to say; but from initial interviews, it seems like he's very realistic, and perfectly knew what to expect at Barca. Maybe that's just all a facade, I don't know.