Assassin's Creed died the minute I knew I had to deal with Ezio again. I felt it would've been better anyway without the present-day nonsense, would've been much cooler to work through the ages up to the present and see how all the events influenced it.
Call of Duty: After CoD4 I picked up MW2, but after a playthrough in 2 afternoons I just felt unsatisfied because I've seen it all before. After reading and hearing about Black Ops and now MW3, I'm even more turned off.
Battlefield: so-so starting to get turned off. I played from 1942 to 2142, then Bad Company 2. BC2 was enough of a change to interest me in the series again after having played BF2 to death. But now BF3 is just more of BF2 but with suckier maps and 10 different red dot and 4x scopes to unlock for each gun seperately. I still like it, but only for quick matches now and then (so probably won't buy the next installment).
Final Fantasy: The games just become terrible starting at X-2. The good games still includes FF11 (released about 3 weeks prior to X-2) which I all enjoyed thouroughly, but then things just took a shovel to the face and even Versus XIII which is supposed to be the new VII doesn't interest me in the slightest.
Half-Life: But who isn't at this point? Does anyone even still care about episode 3?
F.E.A.R.: From the first expansions I just knew this wasn't going to have a very good future. Personally, I don't think this should have ever turned into a series.
Heroes of Might and Magic: Heroes III is my all-time favorite, I also enjoyed IV. But now I have tried the sixth installment and it was painful for me how much everything was widdled down. Things have been made simpler, yet more complicated at the same time for experienced HoMM players. The graphics are nice, but the gameplay is key here. With V coming out I was already edgy, but still got my hopes up for 6, and landed on my face.
So yeah, basically the big titles have gotten horribly stale, or just plain terrible. The one series I do still enjoy though is the Total War series, and while The Elder Scrolls 4: Oblivion was a let down, I still had fun with it and decided to give Skyrim a try (but of course the PC pre-orders got stolen from the store I ordered at, just my luck).