This gen had plenty of great moments/games but way too many series I loved either died, declined, or disappeared completely.
Halo
Started great with Halo 3. Was solid with ODST and Reach, and then Halo 4 came out and killed so many aspects of what made the series great. Halo's future is a big question mark right now.
Final Fantasy
Going from one of my favorite games in the series(XII) to XIII was a huge disappointment. Then there is the whole XIII trilogy stuff, no localization of Type-0, and the awful original version of FFXIV. Fortunately the series looks to be going back in the right direction with XV.
Splinter Cell
One of the best trilogies on the Xbox became I don't even know what. Double Agent was average, Conviction was a horrible SC game but fun action game, and Blacklist was a weird mix of Conviction and old school games with too many glitches.
Ninja Gaiden
I beat Ninja Gaiden Black on Master Ninja. My hype for NG2 was insane, but it wasn't as well designed and the framerate was awful at launch. NG3 is not even worth discussing.
Resident Evil
I wasn't the biggest fan of the old RE games. I enjoyed them, but RE4 was a classic. I was dying to play RE5 and while it wasn't RE4, it was still fun and I replayed it multiple times in co-op. I hated the demo for RE6 so much I never even played the full game.
Ratchet and Clank
The series started off great this gen with Tools of Destruction and a Crack in Time, but All 4 One and Full Frontal Assault were not good Ratchet games at all. It looks like Into the Nexus is a step back in the right direction though, even if it is a small step.
Metroid Prime
Other M. Enough said.
Gears of War
This series has almost died in one generation. Gears 1 was a classic, 2 was very good despite tons of issues with online MP, and 3 was a masterpiece. Judgment did enough damage to potentially ruin the series forever. MS needs to sit on this series a few years and bring it back with a focus on what made the original the classic it is.
Unreal Tournament
UT3 was a decent game, but strayed too far away from what made the original UT so fun. This type of game is pretty much nonexistent now so who knows if we'll ever see it again.
Jak and Daxter
This series just disappeared this gen. I know Naughty Dog took it up a notch with Uncharted and the Last of Us, but there's no reason Sony couldn't have had another developer work on a Jak game. I loved all three of the first games despite their differences and changes. Hopefully Jak returns on the PS4.