I thought of this subject while watching the DVDs for Freaks and Geeks, a great show that, unfortunately, got cancelled after completing only one season, even though it garnered a lot of critical acclaim. I only caught on to the show after it had already been cancelled and reruns were shown on some cable network a few years back and I thought it was a great show. I decided to rent the DVDs to see the episodes again (and see if there might have been one episode to slip past without me seeing it).
If you haven't seen the show before, I highly recommend it. Here's the IMDB.com link:
http://www.imdb.com/title/tt0193676/
Any other one season TV shows that were really good but got the axe anyways?
Arrested Development almost had the same thing happen to it that happened to Freaks and Geeks, but, luckily, got picked up for another season.