Lost would qualify for me. Prime example of a show being dragged out several years longer than it should with no actual plan for where they were going and how to get there. Sliders as well. I love me some sci-fi and it had an awesome premise, but my god all that stuff after the first season or two.
The biggest problem with Lost is that J J Abrams had a hand in it. He has a tendency to create shows and then either lose interest in them or pick up another project to pay more attention to. This has happened with all of his shows and, coincidently enough every one of them got worse as they continued on. For Lost, he left, I believe, midway through the first season to work on MI3, which explains why the show began to go downhill after the initial season. The same could be said for Alias and Fringe, as well as possibly Person of Interest and, if it lasts longer than one season, Revolution, despite serving only as executive producer of the last two.

Lie to Me is one of the examples that comes to mind for this topic. The first two seasons were really good and the last was decent, though it definitely felt like the main character was becoming a caricature. Castle is another one that can be used. It is still enjoyable, but too much emphasis has been placed on the relationship between Castle and Beckett that it has become almost all about whether or not they will get together. The new season seems to lessen that emphasis and hopefully continues to do so (I've only seen the first episode so far). Most television comedies have a tendency to suffer from this as well. How I Met Your Mother is atrocious and not even remotely funny now, because no development actually occurs and the purpose seems to just push the "story" for however long CBS will continue to shovel money its way.