Or better yet didn't live up to initial potential. For me a great example would be Heroes. When it debuted it had such potential that it took my favorite show on at the time from Lost but while Lost regained some direction Heroes lost it's way. Not talking about shows that were great but hung around a little too long, like Babylon 5, because writer told his tale but networks wanted more.
More like shows that initially showed something but quickly lost it. Two shows I recall had outstanding pilots but never reached that level again, where Last Resort, an Adre Braugher submarine series, and the revamp of Hawaii Five O. Both pilots really captured my attention but never did afterwards to the same extent. At least Heroes had one great full season, these shows only had stellar pilots.