The Comeback is a really great show, but I think the reason it only had one season is that it fails as a comedy.
Lisa Kudrow is incredible (as always) and Valerie Cherish is a fantastic character. The series does have its funny moments, but it's a lot like the original version of The Office, and I find myself just cringing through a lot of the it.
Valerie is a very sympathetic character, and I think the problem is that she's too sympathetic. I rarely laugh when I'm watching the show, because I feel way too bad for Val. I just can't laugh at her. She's so nice, and she doesn't deserve any of what happens to her.
I like the series as a drama, and the two shows-within-a-show, "The Comeback" and "Room and Bored" are a great touch and (I think) very original. I can't think of another show that does this. Extras does, but not to the same extent as The Comeback and I find it very refreshing and a pleasure to watch. If only poor Valerie had better luck.