Chuck, great example, well explained
Heroes, agree there as well.
How I Met Your Mother was the most unwatchable tripe I've ever seen towards the end.
Homeland, what an amazing 1st season. What a terrible 2nd season. I don't know what happens after that.
Dexter was really bad after season 4 and Trinity. I even really liked all the guest roles in the show. Julia Stiles, Colin Hanks, Mos Def, Ray Stevenson I thought did a great job, and Yvonne "Sarah Walker" Swarovski Crystals. All great. But the show wasn't. I don't really know whose 'fault' it was. Michael C Hall's performance was consistent from day 1, and I thought that Debra Morgan (don't know the actress) has a nice little metamorphosis. But it doesn't really hold up.
Community had the gas leak year...
I don't remember if Smallville got significantly worse or if I just grew out of it.
Trueblood got so ridiculous. So stupid and ridiculous, it almost seems like "that's the point".