Interesting topic!
Well even though Federer won 3 majors back in 07, I do believe that since 07 he played differently. Granted the way he played at the AO07 was probably one of the best grand slam performances by fed, but if you are watching some videos of the previous major (06 US Open) and then AO07, I do see a difference at the forehand swing.
He played with way more top spin and in addition he didn't went for the big shots if he was far behind the baseline as some of you already mentioned.
US2006:
http://www.youtube.com/watch?v=zcr8fMWpo5Y
AO2007:
http://www.youtube.com/watch?v=rbBo3a5euMI
I read on top comment of a youtube video once that ATP changed the balls back in 07. I honestly never googled it.
Does someone know something about it?
Another thing are definitely the courts getting slowed down. I don't remember when it was, I think Miami in 07 for example.
Everybody is always saying Fed's prime was 04-07, but to be quite honest he definitely played different in 07. Not only the forehand swing, also mentally (beside the majors) he didn't seemed as strong as before. Frankly I think after being as dominant as he was, he probably relaxed a bit at the Masters1000 and was a bit casual. Who can blame him.
As far as Grand Slams, well it was still one of his best years.
Back to the main topic now:
In retrospect, I think he really started to play better on clay. Especially the way he played at RG after the Haas match that was pretty strong and different to the years before. I think people don't give Fed enough credit for that RG title, with Nadal out of the way he really had to feel a lot of pressure. Furthermore, I think the way he played in 10 and especially 11 on clay was stronger then ever. He played really smart and after adding the forehand drop shot back in 08 he really started to play better every year. Yes, I think he played stronger (tactical considered) at RG10 than RG09.
About the serve:
There is no way in hell his serve is stronger then ever. If you are watching some old Federer videos and I'm not talking about highlights, you will see that his mental strength was unreal. Especially on break points or deuce you could always count on him hitting a first serve, heck even an ace. And I can't stress this enough how important that is, but I never saw Fed hitting a double fault on a break point or at crucial moments in big matches, never.
And he definitely isn't as fast as he was. Hard to say when he declined. At the AO07 final he was quick as a cat, especially the sneaking in tactic against gonzo in the final was a perfect example how fast and smart he played back then.
And after winning RG and SW19 and being a father, I think he wasn't really on fire anymore. Or at least not as much as he was when he was 24. But this is just the way it is, i guess.
I think the mental part is the biggest change. I'd say due to the fact that he won pretty much everything a player can way. And on the other side, like I said being a father affects your spirit and getting older doesn't necessarily help as well.