Having the best talent in the world at one of the world's biggest clubs mean that you have the highest expectations of any manager in the world as well as one of the busiest schedules. After what Barca has done the last few years people expect them to literally win everything, and like I said, the pressure and personal attacks from the spanish media are like nothing I've seen in any other country. It's all well and good to say that this shouldn't matter, but it does; no-one is impervious to that kind of pressure and he deserves a break from it. Plus, Barca have already had 3 years of domination and some key players are starting to age (puyol, xavi) so I doubt Pep thinks he's ever going to get back to the pinnacle of achievement at this particular club.
I don't know, it just seems obvious to me that the stress of such a high profile job would get to you after a while. He said himself that he didn't feel like he had enough energy and enthusiasm to motivate the players anymore, so I think in both respects he did the right thing.