While some saw Cesc Fabregas, Alexis Sanchez and Thiago Alcantara as mere squad players, that is just not the case at all. They are indeed squad players, but not your regular squad players. They bring the kind of quality depth that will take Barcelona to the next level and they'll all feature a lot.
Last season, the likes of Xavi, Iniesta, Messi, Villa and Pedro had to play pretty much every match and some of them were visibly tired towards the end of the season, namely Villa and Pedro. These players had to play almost every game because their replacements weren't quite of the same quality.
This year, that is very different. Cesc and Thiago's presence means that both Xavi and Iniesta can get plenty of rest without a significant loss in quality. If Xavi and Iniesta were great when they had to play 50-plus matches per season, just imagine how much greater they will be now that they will get adequate rest.
Barcelona's midfield has just become even more potent—a scary prospect for opponents. Last season, Keita was the only viable alternative to Xavi and Iniesta, and the Malian isn't exactly known for his creativity. This season, he's being used as a backup for Sergio Busquets; a position that might suit him much better.
Up front, Sanchez's arrival will allow Villa and Pedro to have the rest they couldn't have last season. Even Messi can be rested, with Sanchez and Cesc both capable of replacing him decently.
Besides, all these players bring certain qualities that the squad didn't possess.
Thiago and Cesc are more direct and have a better eye for a goal than Xavi and Iniesta (in fact, they both scored in Barcelona's recent demolition of Villareal); they offer a different kind of threat. Alexis Sanchez is a much superior dribbler to Pedro and Villa and might be very useful unlocking the parked buses Barcelona will inevitably face. We can't forget Ibrahim Afellay; when he returns from injury, he will be a valuable rotation player at both midfield and attack; his versatility will come in handy.
Overall, the squad got stronger and deeper. Even if, for instance, Xavi, Iniesta and Villa were all injured at the same time, Barcelona would remain a magnificent team, favorites against anyone.