Maybe this post is a little on the late side, but I just have to say I really enjoyed episode 13. Usually I would have to describe my K-On! watching as a common knowledge thing, as I have never been overly impressed by the humor, as it often is the same joke over again. But neither have I ever been really bored during an episode, and I'm certainly not of the opinion that it is a bad show, just not especially worthy of the attention. But when is that ever the case with popular things?
Regardless, about episode 13. What starts out resembling a usual side-character focus show, becomes an uncharacteristically eventful ride(being K-On!), especially during Azusa's dreams, and you are left wondering throughout the episode about what is dream, and what is not. You also get to see the main four in more of an outside perspective, which had been done before in the series, but not to this extent, when Azusa considers her feelings about them and her relation with them.
Another aspect that was interesting was to see Ui's relation with Yui from Azusa's side, rather than from that of Yui herself. This is something that has also been done before, but this time it had, though done in a funny way, some serious undertones, when you see Ui reacting pretty harshly to criticism directed at Yui. A sister-complex indeed.
Maybe nobody else noticed a difference in pace and story, but those are my thoughts, at any rate.