You gave some great examples there, Tim. The first four seasons of Dexter compared to the last four are like night and day. One of the best shows I've seen and one of the worst. Season 4 was almost as good as the incredible first season, and Season 5 was a massive let down right from the start. Julia Stiles sucked, the kids were written off the show unrealistically and everything became super contrived. There were more stupid side plots and everything just got lame very quickly.
House of Cards was never as good as the first few seasons of Dexter, but I liked it a lot until this year. I only finished half of Season 3 and wasn't bothered continuing. It just wasn't very entertaining. The characters (and the show itself) kinda lost their balls.
LOST is another example that occurred to me. Between Season 4 and Season 5. LOST was never a great show in my opinion. It just had moments of brilliance, and was a great show to talk and theorise about before we realised they were making it all up as they went along.

However, the show was always pretty fun to watch until Season 5 where it took a serious nose dive. Too much focus on shitty characters with no charisma like Juliet. Too much character flip-flopping from the likes of Kate. And too much supernatural shit. Having a little supernatural stuff in the show - shrouded with mystery - is what made it so good in the first place. But they went totally overkill in Season 5 and it made everything less special.
Veronica Mars is one of my favourite shows and Season 3 isn't bad. It's really good at times. But it pales entirely in comparison to the first two seasons which were almost perfect. It went from being one of the best shows on television (arguably the best), to just being a decent show.