It is an almost univeral law that for any TV series you care to name, the second and third seasons are the very best the show has to offer. (It's not absolutely universal, before you start citing exceptions...)
The reasons for this are quite simple, actually. In the first season, the creators are working without feedback, and so they have to take their best guess at what the audience will like. So, some ideas will work, some will fail, and others will do okay. In the second season, then, they are able to refine the concept. Ideas that didn't work out are dropped, while others that worked well are expanded. And so things are great for a year or two.
Problem is, after three years or so, they will have used up all of their best material. At this point, they will go one of two ways - either they start expanding on lesser ideas (resulting in an inevitable loss of quality), or they will try to take the show in new directions by shaking it up (almost certainly resulting in a loss of quality).
The latest casualty of this law has been "Fringe", which ended its third season extremely strongly. Unfortunately, they've started the fourth season by introducing a major paradox that has dramatically shaken up the world and the characters. The reasoning between these changes makes no sense (in order to fix a broken universe, some of the characters deliberately caused a time paradox?). And, worst of all, they've retconned the previous three seasons. Effectively, it's like jumping into an entirely different show four seasons in, but only after someone managed to destroy all of the DVDs of the previous three series.
Basically, it's not a good situation. As things stand, "Fringe" will be joining "Torchwood", "Bones" and "House" on the reject pile, which is a shame.
Meanwhile, "Merlin" is continuing to be quite good, although it, too, is suffering from Fourth Season Syndrome. (I would say more, but Lady Chocolat hasn't seen it yet... spoilers!) "Strikeback: Project Dawn" has ended, but was at least good fun, if not particularly high art. "Carnivale" remains good, but is nearly finished. And "Terra Nova" remains... okay, I guess.
Finally, two annoyances. It looks like Sky have taken the decision over the last season of "Chuck" out of my hands - they've decided not to bother showing the last thirteen episodes. I'm marginally annoyed at this, since I had decided to see it out. And it's still not clear when (or if) "Clone Wars" will be returning. But then, that's going into its fourth season as well...
As things stand, it looks like I'll be down to "Fringe" and, maybe, "Terra Nova" by the end of the year (with "Nikita" returning in April-ish). Given that neither of these is currently very good, that may finally be the time to drop the Sky subscription - it's been a long time coming.