I just think this performance is so fun, seeing everyone dancing together and being happy (except for Rachel, not sure what's up there). Love seeing the Cheerios together, too.
But mostly this performance is notable for the rarity that is Kurt being the first one to join in the dancing. A lot of times he holds back or flat out doesn't participate in these impromptu numbers, and it's been really noticeable this season, particularly when he moved away from everyone during Puck's performance of "Only the Good Die Young" in Grilled Cheesus and Carl's performance of "Hot Patootie" in The Rocky Horror Glee Show.
I hope there's some explanation for the complete 180, even if it's just something like Holly being a better teacher than useless/creepy Will. Because Kurt being this adorable and enthusiastic has so not been the norm lately. I love him getting sassy with Finn and then later silly with the holdout Rachel! Cute.