I know you don't put too much on coaching but I think Moya deserves some credit with Nadal's improvement on serve.
No I completely agree.
My stance on coaches isn't that they can't impact a player's game. That'd be stupid. Of course they can and they do. However, I think that their influence is ultimately limited. There's so much that goes on during a tennis match that's just solely on a player's racket, so a lot of times if a player is going through a bad patch the de facto reaction for many of us is "he needs to change his coach/bring in a new coach."
I also think that a coach's influence is typically at its most effective early on, where the new ideas/tweaks are sensed just by watching the player do new things and improve on them. The serve, in Nadal's case, is a very tangible improvement. It's actually one of the easiest improvements to spot in tennis because it depends purely on him and not on his opponent. And given that this year has been by and large, the best he's served throughout his career over a long period of time (as opposed to small stretches like the 2010 US Open), it's safe to say Moya has a lot to do with that and I don't think it's a coincidence.
On the flip side, I think a coach's effect wears out over time, and that's why we witness so many coaching changes in tennis. There's only so much they can do. That's why I think their effect is largely overstated.