I don't watch much TV and my two I came in here to mention have already been discussed- Lost and Doctor Who.
Lost... if only they didn't have any supernatural stuff going on. Some kind of secret conspiracy involving the scientists on the island would have been fine, but smoke monsters flying around and time traveling was just ludicrous.
With Doctor Who it fell apart after Tennant left, everyone knows that. The first couple of series of Smith I gave it a real go and didn't want to be one of those people complaining, but it was undeniably worse. From that point I started to find all of the characters irritating, the stories boring or too weird and usually when I've tried to watch it in the last couple of years I almost always end up doing this halfway through: