lol @ anyone saying FFXIII was a disappointment. Amazing game was amazing. Best battle system in the entire series, with only X's coming remotely close.
The only other 2010 release I played was Starcraft II, which was fairly disappointing in terms of story. I still had fun playing it, so I can't really call it a let-down.
That said, I have to say this year's most disappointing game was Runescape. Anyone who plays will know, but I'll elaborate. Until two years ago, this was an MMO that had ALWAYS received WEEKLY updates. Sometime in 2009, the company in charge of the game, Jagex, decided that they needed a week off, "No Update Weeks", each month to laze around having tea and crumpets (they're brits, you see). This was bearable so long as they stuck to a schedule of 3-4 updates per month (the number of Tuesdays minus one [Tuesdays were their regular update day]). This year, they just crumbled as a responsible company. They decided that each month gets only 3 updates regardless of the number of weeks (Tuesdays) in it. They decided that any time there is a bank holiday, for even one day out of the week, no update that week (this stacks with No Update Weeks to make 2 planned updates and 2/3 planned No Update Weeks in a month). That fucking killed any rhythm of new content that kept the game continuously entertaining. Here's the real kicker though: Any update that looked promising was "unexpectedly delayed". Calling it "unexpected" is a joke because it's been pretty much guaranteed that any good update will be delayed by a week. Jagex usually pinned it on some vague, undefinable trouble that the update was causing, or problems with a background update that no players had been told of. Those both translate into "We're incompetent as fuck at making this game, so all the players can fuck off while we drink our tea." I was expecting a culmination of holidays, delays, and No-Updates to lead to the epitome of laziness in a 1-update month. That didn't happen, but they tried damn hard in November which was a 5-week month that got two updates (one of which was on the high-end of unusable which I'll explain below).
If you don't see anything bad with this so far, there's still the content of the few updates - I barely need two hands to count the amount of good/usable updates from this past year. Over half the year's update were targeted towards, not only low-level players, but beginners. Updates that add absolutely no value to the game for PLAYERS that add absolutely no value to the game. The level difference being so vast, these kinds of things are not worth my time at all, and are thoroughly unenjoyable. Ironically, we got a couple updates that are so far on the other side of the spectrum, I can't even do them with my character that is ranked in the top 70,000 out of millions and has 145 days of play time. Days. One requiring all skills in the 90+ range (which requires about 500+ days of play time and a true no-lifer), and another requiring 95+ in a skill that only came out six months ago and earns some of the slowest experience on the game (it goes much faster by sharing the load with four other players, which is such an illogical, rage-inducing factor that it requires it's own post). Furthermore, they just do really stupid things. They take time out of their schedules to do community events like meeting up with clans and other asinine bullshit which is ironic because despite that playing time, none of Jagex actually know anything about their own game as exemplified by so many updates that are just plain broken and uncalled for. One such update made me lose any and all faith in Jagex - Climbing Boots. They arbitrarily decided that an item (of which there were millions of in the game) could be sold for 3750x what they had paid for them. This is in a game where inflation is rampant, and the money is becoming more and more worthless each day. What's worse is that the people that naturally had a lot of this cheap item were PKers who are absolutely the worst kind of scum you can meet on the internet.
So yes, Runescape has had a bad year. Usually during November/December Jagex releases the best updates of the year to kind of make up for other bad updates. Not this year. We got some plain dull updates, and one that had potential involving updating armor to absorb damage as opposed to just negating it. They ruined that by making it only applicable to PVP (the cesspit of Runescape filled with botters and 13 year-olds) and boss fighting (which is utterly dominated by those no-lifers with 500+ days playing time). Jagex really screwed the pooch this year. I can't imagine anyone that's truly satisfied.
tl;dr Jagex used to be a good company, but has deteriorated to shit-tier and is trying their absolute best to ruin Runescape.