TV shows that started great but went downhill
For me...
Dexter - Peaked at the season four finale, went on for another 4 seasons, ultimately giving us the worst series finale in a long time.
24 - The early seasons were fresh and intense, sadly it became more difficult to suspend disbelief, and the reboot, LAD was just an insult.
Fringe - Season 1 and 2 were great, season 3, 4 and 5 however werent, it wasnt as bad as some, but it had potential to be far better.
Heroes - While never great, season 1 was good, from there though, things went from bad to worse.