After thinking about the episode(and watching it again) I'm starting to draw some conclusions about the new episodes that might explain why these Rarity(and Spike in this case) centric episodes have been so good:
In season one all characters had arguably their own respective episode showcasing their attitudes and personalities in front of situations that provided a better insight on them, this was reinforced by their harmony element and their attitudes among other ponies. In this season however there is more attention to some situations and not the characters, as I mentioned before lately canon has tried to fulfil some of the fanon demands(Luna most as the more remarkable example), this attempts have ended in weak episodes that lose sense of interest at not having anything to offer about the main characters.
This and the previous episode showcased a perfect example on how an episode should be written, since these episodes are character centric their emphasis is on providing more information and facets about the characters rather than creating a sitcom, likewise most of the impact of these episodes is directly related to how the characters reach certain situations and how their new facets explored (in Lesson Zero for example we can see new Twilight facets, and the puns based on this new facet).
Yesterday's episode showcased more exploration on Spike's character, and most importantly it showcased new characteristics of his persona, here is also explored how its surroundings respond to this new facets, and that is what makes the episode entertaining to watch, something that was absent on the Owloucious episode(whatever name that episode has) and something that has become less common in this season.
I guess that other part of this trend is the fact that Rarity was never explored at the same level of TS/RD/AJ in season one, and due to her character is hard to define some of her characteristics when so little attention is given to her (and keep it mind that Rarity is probably the most stereotypical of them all, making the task of characterizing her even more complicated). Likewise Spike actually shows huge steps in character development and some his new facets are showcased, which gives him a larger field of interest which surpasses whatever interest that might be given to overused characters like RD and FS.
That's pretty much what is running on my mind right now, but is quite late.
Goodnight everyone.
