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.
IMO Djokovic was playing at his own limit at times last year. This year, he plays mostly within himself, and when he tries to push it to that limit, he has often made mistakes. To me that is normal in my experience (more so than the opposite, though it does sometimes happen). Not being able to bring ones very best in the biggest matches is also normal. It is rare to see somebody come out and play at their limit in an important match but Djokovic did it at times last year. This is not to say he has a history of playing poorly in important moments, as you said, he has a good record in areas which are seen as indicative of mental strength. I only mean it is rare for a player to show their best in an important match (these are often remembered as players 'best' because it is combined with importance). I certainly agree that Djokovic has had his second best year. That is hardly even up for debate at this stage.
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.
This is an interesting one. Djokovic knows how to beat Nadal on clay but it is still probably the toughest match on tour. Yes he has the ingredients. It remains to be seen how both are playing during clay '13 to predict whether or not he can put together the kind of form required to win against him again.
I agree that his match-up with Murray isn't favourable but still, it's at worst 50-50 imo.
Yes. It's not that it's a terrible
match up for Djokovic, he also neutralises Murray's aggressive shots to a pretty large extent as well. It's very awkward for both guys really. Neither has the attacking game to consistently dictate successfully throughout the duration of a match. The risk-reward judgement which makes it either worthwhile or not worthwhile to be aggressive really hangs in the balance in that match-up. It provides very interesting points but a lot of errors to finish rallies (which a lot of fans find unacceptable for a big match) because finishing off points is hard for both against the other. The reason Murray has a quite good record vs Djokovic (considering he has been the lesser player for almost all their matches) is because he has the better serve which is the one steadier weapon which can be relied upon to do damage.