The difference in mediums.
Well it's relevant when discussing one's subjective appreciation for one compared to the other. If you have a bias in favor of one's strengths, it will be difficult for you to ever prefer the other. So for me, no matter how good television is, if it's working within the confines of the medium as currently understood, it will be extremely difficult for television to overtake film in the eyes of someone like me who goes to motion picture mediums for mood and atmosphere. Television is simply prevented from doing that given the current restrictions - commercial and otherwise - on the medium.
Probably doesn't sound like it, but I'm agreeing to disagree. If you prefer plot and story, television is clearly telling more compelling stories. If you're looking for something more focused on visuals and aesthetics, then film will win out.
The problem with TV IMO is a show will start off pretty good, then realize they have a hit and try and drag the series on for 10 seasons completely ruining the story line. (Heroes and Lost come to mind) a movie done right has a much better story as atleast they keep on track and end it.
This is one of the commercial limitations on television I was thinking of above. But I'm definitely able to get over this in the same way I'm able to get over a botched ending to an otherwise great movie. Sure, it might get docked points/stars/whatever in my mind, but a bad ending to a movie or few seasons that were clearly a money grab will not, for me, nullify the quality of the movie or show that came before.
Compare, for instance, Breaking Bad and The Shield. Breaking Bad was probably the more consistently great show throughout its run, but The Shield stuck the landing with one of the best finales of all time whereas Breaking Bad's was, in my opinion, a complete clusterfuck. I will still put BB over The Shield 90% of the time when ranking television shows.
Then again, I'm also one of the few who, after some time, has come to love the Lost finale (the whole season and the actual finale episode).