The Crucible entity is a literal and figurative Deus Ex Machina - its existence isn't foreshadowed at any point over the three games, except perhaps the fact the Illusive Man's plan requires Reapers to be controllable, but even then, nothing implies they are
being controlled by an external force. It's an utter asspull performed in the last five minutes of the game: Suddenly a sparkly child appears, performs some exposition and you walk to whichever endings you've earned (And seeing as I was playing Singleplayer only, I got forced into the destruction ending because my war assets only counted half. Brilliant move, Bioware...)
People complain that it doesn't provide closure to the other characters, but I thought that was the point of talking to them on the planet. would have been nice to see how they cope without the mass relays though, or have Joker chew me out for killing EDI (wait, he can't, Shepard dies too) so I have no issues with it on that score.
also if the whole series is a story being recounted by a father to his little girl, does that mean he told her about all the sex? 'cos I'm not sure how I feel about that implication.
The From Ashes DLC is very worth it, even if just for the Prothean particle beam. The squad member himself is quite good, the best part of Javik is his Dark Channel power, which you can get as a bonus power (and, on a replay, start with).
Oh, and EDI being the Overlord AI was a nice touch
Definitely the weakest of the 3, but still better than most games
There's no way Bioware are going to alter the ending with DLC, at least not significantly (although it would be the fastest selling DLC ever)