Supergirl got the casting right, but the writing was awful, and the special effects were BAD for the first half of the season. The second half shaped up considerably, but the damage was done there. The biggest issue has been the writing, specifically the pacing. The first season packed in:
-introduction + skill mastery
-Fort Roz emerges, gets defeated
-Aunt gets introduced, killed
-J'onn Jonzz hiding in DEO
-Maxwell Lord scheming
-romance between Kara/Winn, Kara/James, Kara/Adam Foster (which is the worst name for a character whose main personality trait was "adopted from Cat Grant". It's one step removed from Joe Piper the plumber.
-city accepts Kara/rejects Kara under Kryptonite/accepts Kara again
And I know I'm leaving some out. I hope CW smooths things out some, because Melissa Benoist IS Supergirl - it'd just be nice to see some good writing backing that up.