Let me say upfront: I didn't actually like it.
* I feel like it was missing the show's frenetic energy. This is a show that usually has three plots in 23 minute; this was an hour-long special that only had two (the boys' recordings, Katie and Bitters) and I guess a half (Mama Knight packing). It was pretty clearly built around the two guest stars, and I was ambivalent about them. (Don't hate me, internet, but I... really don't think Miranda Cosgrove is a very good actress.)
* Tone-wise, the episode didn't match basically the entire rest of the series. There were fairly few random sight gags and cartoon sound effects, recurring background characters, etc etc. They seemed to sacrifice the tone for the Christmas-y-ness, which I don't think was worth the trade off.
* The plot of every Christmas special ever is "We're not going to have Christmas! ... wait, yes we are!" ... Sorry, I just expect more originality from BTR. The thing about it being really good is that I have actual high expectations for it, and this didn't meet them.
* The biggie: I'm usually pretty tolerant of Christmas specials; they aren't my favorites, but they rarely bother me. This... did. The moral wasn't any of the generic "Christmas is about family, not presents," / "Christmas is about love and good will, not presents," / "Christmas is the time of year you try to be a better person," style messages you usually get. It was pretty clearly, "Christmas is THE BEST THING EVER and it's TOTALLY OKAY to force it on people who don't celebrate it the way you do!"
Yup, that plays directly on my personal issues. We were the only Jewish family in my town. Repeat: only. As you can imagine, that makes the holidays, uh, awkward. All of my friends celebrated Christmas and looked at me pityingly when I explained that I didn't; we sang Christmas songs in Girl Scouts and school chorus; all TV, movies, commercials, etc were about Christmas. And I get that it's enough of a dominant culture thing that the default for non-religious people is Christmas, and for a lot of people it isn't about religion at all... but when you're the only ones around who don't celebrate it, it feels weird and othering.
So the Katie B-plot here really, really didn't work for me. Bitters doesn't celebrate Christmas. Sorry, Katie, but that's not an invitation to tell him he's wrong for that, or that he not just needs to do it, he needs to do it her way. There are plenty of reasons not to celebrate Christmas, starting with (but not limited to) the fact that not all people are Christian. It feels awkward enough to be Othered about it for a month each year to begin with; having someone directly confront you and try to force you in to it isn't compassion.
Obviously that's not the show's POV -- in the show's universe, Bitters was just waiting for someone to take it upon herself to save his Christmas and he really did want it. Fine. But that attitude of the show's, that someone who doesn't celebrate Christmas just hasn't seen the light yet and must be saved from that, is offensive in and of itself.
* I was also surprise there wasn't at least a nod to the fact that not everyone celebrates Christmas. I feel like most shows at least do that, with a token nod to "Merry Christmas or happy Chanukah or Kwanzaa or generic winter celebration!" This really, really didn't. Not a single character in the BTR universe is Jewish? At all? Really? (And that's without getting into even lesser recognized groups, like atheists and non-monotheistic religions and others that I am privileged enough that I haven't had to think about and thus can't name off the top of my head.)
The episode had some funny lines and good moments, but as a whole it really didn't work. If it hadn't really upset me with the message, it still would have been weak; as it was, it took over from Dance as my least favorite.
So in conclusion: bah. Humbug.