Roseanne should have ended about 2 seasons before it did. They clearly were running out of ideas. The once great dramedy turned into over-the-top farce.
Dallas should have ended after about 9 seasons.
Without a Trace went on about three seasons too many. Cold Case has been run into the ground and should have ended years ago.
TNG should have ended after season six. Season seven was middling.
Melrose Place should have ended after season 5. After that they just retread stories and had a few heavy cast turnovers and by then most of the fan favorites had left.
Along the same lines The X-Files should have ended after season five. I didn't care for the most part for the last 4 seasons even moreso after Duchovny left and Anderson basically made cameo appearances in the 9th.
Heroes after season two.
nBSG should have just run two or 2.5 seasons and cut out all the useless filler as well as the aimlessness of its final season that existed solely to stretch things out.
It seems these days that shows either die before their time or go on far too long. A natural death is truly a rare thing in modern TV.
The one series that looks to defy both those scenarios is LOST. The writers actually look to have designed the show to run for just the perfect amount of time. Not peaking too soon and declining over the years and not ending before it peaks.
Most shows really can't say that.
None. If a show bores me, I stop watching.
Not for me. If it is a show that I enjoyed for several seasons and was at one time a good show, I feel compelled to see it through until the end no matter how bad it becomes. Now if it is a show that I never cared that much for or it never caught on then I'll quit immediately.