What they got the awards for is kind of irrelevant. Again videogame stories are graded on a curve, especially when it comes to games journalist who spend the vast majority of their time speaking in hyperbole, and empty group think.
In other words, they wouldn't know a good story if it bit them on the bottom.

t:
I am talking about the entirety of their storytelling. the impact on the game you play. You are thinking about this in a very linear manner, when the entire point of choice is the exact opposite. You brought up choice. My point is that actual choice changes a game on a more fundamental level then who is doing the talking and shooting when you just end up playing the same game anyways, just with skin swaps and different pictures at the end.
A fantastic example of this done right is the Witcher 2. A game where depending on your choices the game and story you play itself changes. Depending on what you actually chose to do, you end up encountering completely different missions, characters and overall storyline. The Witcher 2 is two games in one. More if you considering just how different you can play each of those paths.
It is not just about the ending of the game if someone dies. Think about how Mass Effect is structured. Now consider how important a lot of your companions become in the grand scheme of the galaxy. And yet their deaths do not actually matter.
Their deaths should change the fundamental storyline. If Wrex is dead, why are you playing the same mission as if Wrex is alive? So on and so forth. If Wrex is that important, why can he just be replaced by another character and the story still function in the same manner? You do the exact same thing whether he is alive or dead.
Which brings me back to what Bioware does well. What matters about the characters is their personalities. Whether you like them or not. It has nothing to do with the actual story, its structure or how it is told.
It makes choice superfluous and is actually bad storytelling.
How does this change the game's story, the game you play, in anyway?
It does not address any of the problems with the original ending. It is an epilogue, that does not effect the story mechanics one bit. It is literally tacking on an ending that does not address that actual flaws in the storytelling leading up to it.
If the intention of the game was to end a certain way, that is perfectly fine. Portal does that and I think it is storytelling nirvana. But Mass Effect is suppose to be about choice and how it changes the game. It doesn't.
It also doesn't help that Mass Effect 2 and 3 are poorly written, hilarious dialogue not withstanding. All the endings are bad.
But why does it suck? I could go on for days about what is wrong with the storytelling in the prequels. I am explaining the inherent flaws I find with Bioware storytelling using examples from the games themselves and storytelling in general. In your defense, you are bringing up awards and the fact that you enjoy it so much and admire them as storytellers. Which is great, I enjoy it to. But this doesn't really tell what makes their storytelling good.
I never played Balder Gates or Neverwinter Nights, so I don't know if it was there. But that stuff actually goes back to KOTOR at least.
Dragon Age doesn't have the morality meter, but they still present a similar issue, though it is far less flawed.
Of course. But what is the point of debate if you aren't going to debate?