I think it's the fullbacks + formation that are making Stones play so well. There was one point today when someone went in behind and Stones had to go defend in the right back area, something we saw dozens of times last season, but instead of just backing off and backing off, he just went in strong and barged him into touch.
The difference I think is he knows Danilo was bombing back to cover, not to mention the other 2 CB's are still there, he doesn't have to stall and wait for Zaba/Yaya/Kolarov to get back before he can make a challenge.
I really want Mangala to come good, but I didn't think he looked good today, his touch is seriously lacking, just about acceptable until pressure comes on him, but then he's still just as poor as ever, and in possession, his passing leaves so much to be desired, he just doesn't make the incisive forward passes the main 3 do.