Most people disliked the ending because of how inconsistent it was with a number of things. Uninformative is a good way to put it. My main problem is with the relative lack of closure I was given with the ending, considering the number of decisions I made throughout the series. I could go on for hours, but I rather not spoil for anyone else who happens to have not played the game (but intend to) and happen to visit your profile xD.
Just finished ME3, btw. I don't get why everyone was upset with the ending. I thought it was rather good, if only a bit uninformative. That may actually be the reason people are upset...right? Either way, pretty great game.