First, it did get kinda dark with everypony except Spike thinking Thorax to be an enemy. Good thing this is a show where things usually turn out, and Spike got his own song (last song he was a part of was The Failure Song).
Second, good thing biting tails isn't completely gone or Spike would have ended up like Chrysalis! Oh... right. Well, you win some, you lose some (though clearly this was in the name of fan pandering, every single fan who said they wanted her to survive granted her immunity to long falls and saved her from ending up like Gaston, though in turn this saved Thorax as well). They'd better bring her back in the final now.
I might add that this is the second episode in a row that did something that if I'd seen it four years ago, it'd ruin the whole season for me, but I can accept both as long as Chrysalis's return sees her destroyed, imprisoned in Tartarus the same way as Tirek (whether or not that is part of the first), or, if nothing else, reformed. (Though this does lower A Canterlot Wedding to just below Princess Twilight Sparkle on my favorite episodes list)
On the bright side, this makes me forget any traces of possibly disliking Rainbow Dash after the previous episode, even though I'd mostly gotten over it already.
Third, this opens up a whole new set of questions. Why haven't Chrysalis and the changelings attacked for four whole seasons? If they don't return, are we to assume Thorax actually reforms them? (the rest can be discussed here or here)
Fourth, so if nopony thought there were any good changelings, what about Kevin? He is a friend of Matilda, but they seemed oblivious to his existence (though the colts and fillies seemed afraid of him). Then again, I guess the Mane 6 didn't actually get into the wedding (can't remember if Spike was there or not), and Cadance and Shining Armor were the ones most affected by the changelings so it stands to reason that they, the Mane 6, Spike, and the Crystal Empire would think all changelings were evil.
On that note, was Kevin the same as Thorax when he hatched (on that note, was kinda cool to see a glimpse of the changeling kingdom)? Did he find redemption through friendship with Matilda after ACW? Or did he simply leave Chrysalis before ACW? This still explains nothing about him beyond what the writers said
Fifth, Starlight appears where she isn't the main character, which is rare. I guess the summary did refer to Starlight instead of Sunburst, even though he did appear. Which is pretty choppy, especially since they spelled it "changling".
Was a bit rushed there, so I forgot this line: Overall, was a pretty good episode, though not my favorite of the season.
(Note: The Sunset Shimmer thing I refer to was her "creature" form being a demon)