I disagree. His backhand was horrendous for much of the match - that is, until Federer fell apart in the fourth set and Dimitrov got a surge of confidence from it. Federer lost this more than Dimitrov won it.
Dimitrov's dumbstruck look about winning tells you all you need to know. I was not impressed much by Dimitrov because I didn't really see any improvement from what he has demonstrated in the past.
I'm afraid this not a case of Dimitrov being better but of Federer being worse.
Federer played bad, that is true. But Dimitrov had his virtues. The backhand was not that bad -- if it was the number of UFE's would be higher. He kept a decent pace on his shots throughout the whole match. His serve was pretty ok. Even if Federer's approaches were kind of suicidal, he kept passing him with efficiency. A lot of guys would have panicked.
He started slow and payed the price losing the first. He won the second by his merits. He was outplayed on the third, but started the fourth strong, and managed not to be broken -- even if that fourth set is one of the strangest sets I saw Federer play. Then, ok, the fifth was a mere formality.
So, in the end he was able to beat B- Federer in a major quarter final. It is still a feat, specially to a guy who was outside top 50 for the whole year.