I think the general atmosphere around the club helps a lot of players. Staff, fans, and squadmates alike tend to buy into the family culture the club seems to embrace: we're (most of us) more patient with players than a lot of clubs, where people are on players' backs if they don't hit the ground running and impress from day one. That takes pressure off players, knowing that the club and those around the club will continue to back them if it takes a bit of time to settle into the squad and way of playing.
Players who don't feel under pressure can play more freely and some players really benefit from that.
Some obviously don't... some players need pressure to thrive, and so not everyone we sign makes a success of it. Players like Keane, Powell etc. who've had a hard time at former clubs, though, are the ones who probably thrive most at Latics, since they'll take a while to fully get the monkey off their back and re-find their mojo; we give them the time and space to do that where other clubs don't; we see a lot of players 'bounce back' at Latics, and I'm convinced that the club's atmosphere has a lot to do with it.