I wonder whether it depends on how emotionally invested I am in the canon. I was having fun with Glee before Blaine showed up, but I wasn't really invested, so I could cheerfully embrace the new guy. The more I care about the existing characters, the more likely I am to resent new characters for taking screentime away from the ones I'm invested in.

I think this pattern holds for me too, and maybe even applies to more than just characters. Like recently I was talking about a show whose first two seasons I loved deeply, but that really went downhill in season three -- except that's really a huge oversimplification. The later seasons brought in a great f/f pairing and heavier science fiction themes, which are both things that I love in in their own right! It's just that the show was no longer doing the same things I'd loved so much to begin with, and I felt the loss of that too much to fully appreciate the new things it was doing instead.

(I am really trying to come up with a premise for a Glee/Uncharted crossover now. I guess there's no reason Cassie can't be interested in musical theater! It could be kind of fun to set Nate and Elena's completely different brand of shenanigans against the romantic drama of the Glee adults...)