I've just finished playing through Mass Effect 3, and I really enjoyed 90% of it. But like almost of the entire internet, the ending was just abysmal. Abysmal writing, abysmal plot, abysmal closure. The ending was such an epic disappointment, and like many others have mentioned, it's mostly counter to what the 2.9 games leading up to it were all about. I'll attempt to write this entry devoid of any big spoilers to the ME3 endings, but no promises if you are spoiler allergic!
I belatedly got into Mass Effect at the start of last year. At the time I was a little bored gaming-wise, and Mass Effect 2 was cleaning out all of the 'Game of the Year' awards for the previous year.
After playing through the first two games back-to-back, I was hooked. I wondered how I managed to not to play them earlier, as generally being a sci-fi geek it was right up my alley. I loved the universe, the plot, the characters, and also very importantly the choice. What the player chose had ramifications for all the plot and characters after it.
I unwillingly let Wrex get killed in the first game, and it's haunted me and my character for all the subsequent games, and also damaged my relationships with all the Krogan race since.
That's what ME3 was continuing so well up until the last 10 minutes, when it was all thrown out the window. If you want specifics on what was disappointing about the ending: this video sums it all up perfectly: 10 Reasons We Hate Mass Effect 3's Ending (spoilers). Almost all my issues with the ending(s) are there in that video.
The bewildering thing about Mass Effect 3 is that, minus a few cliche overtones and forced multiplayer gameplay for single-player reward, it was an absolutely fantastic experience leading up to the last ten minutes. In ten minutes at the end, it almost ruined 5 years, 3 games, and 100+ hours of gameplay.
There is hope though: the Bioware CEO has acknowledged the endings and all the negative reactions, and that they are "planning to directly address it". More info will apparently be available in April.
The cynic in me suspects that this was all part of an evil Bioware/EA plan to upset fans of the series, as a way to make us buy extra DLC following the game's release in order to get a satisfactory ending...