What happens in the ending?
Essentially when you make it to the very end, the game presents you with 3 choices, and you pick one of them. One of the complaints in regards to the ending is that "the choices are all the same". It sort of seems that way at first since the resulting cutscene after making your choice looks nearly identical for all 3. They aren't truly the same though; the game explains to you beforehand what each of of the choices results in.
The real problem is that you don't get to see the results of your choice. Making decisions that effect the storyline and how the game goes isn't unique to Mass Effect--plenty of other RPGs do this--but what is unique to the series (as far as I know anyway) is that your choices affect the future games, you have to deal with the long-term impacts you might not have foreseen at the time. This has always been a major part of the franchise. Given that this is the end of trilogy, there is no next game to see what happens, so the logical thing to do would be to provide a few scenes after making your final choice to see what happens to the galaxy.....yet they didn't do this in the original ending. You're left wondering "what happened to my friends on the Normandy?", "what exactly does galactic society do after the war?", "did I really make the right choice?". There's no closure really.
The new ending, the "Extended Cut" as they call it, remedies that to some degree, as it shows and talks about some of what happens after you make your choice (although it seems not as much as I was hoping for). It also provides a 4th ending where you refuse the 3 other choices and leave, as well as some extra dialogue and changes to some of the original dialogue in the final room to try and clarify, since the game pulls a slight twist (which is something I think was a good part about the ending in general) when you get there and it can be slightly confusing at first.
There is one personal, and so subjective, gripe I have with the ending that both versions share, which is
A few other small issues with game, but still a good experience. Now I just have to decide what to do about DLC for 3, and when I should do another playthrough of the trilogy.