The main problem is he hasn't shown the very best of his form in the most important matches. Whether you think of that as a deviation from his 2011 level or a return to 'the norm' is really a subjective thing. Personally I think rather the latter and as such this year is fine for Djokovic.
I agree with everything you said except that : how could not playing his best in most important matches be a norm ? esp. for a player who has great stats on breakpoints and also in tiebreaks.
That's why personally I think the former of your two versions : 2011 has probably been exceptional, esp. on clay as you said, but his overall level this year still has been much better than any previous year, his serve has even been better than in 2011 and nearer to 2008, and his stats on return are still much better than in previous years although not as good as in 2011.
Besides, as far as his match-up with Nadal is concerned (which was very important for him in 2011 and may be very important next year as well), in their matches this year, I could clearly see the ingredients which made him beat him constantly in 2011 and he made unnormal errors in Roma and Monte-Carlo, and it was tight in Roland-Garros.
I agree that his match-up with Murray isn't favourable but still, it's at worst 50-50 imo.