Actually Neko LLX, I didn't want to accidentally crib ideas from another writer so I
was kinda avoiding it. But since my own story is getting off the ground I don't think that'll be a problem, so I went and read it.
First off... the huge black wall at the end of the world is interesting, and having Rainbow Dash disappear at the end is a great way to give the reader an emotional investment. In short, you have a very sound idea.
There are some weaknesses though:
First, the opening letter to Princess Celestia. Try as hard as I might, I could not imagine that Twilight Sparkle dictated that thing. First, there's the general
format of the latter. You have it bunched as one big paragraph. In actuality, letters are like this:
"Dear someone,
Blah dee blah dee blah.
Yours sincerely,
someone else."
Trust me, this stuff is important. You can have the letter be italicized if you're afraid people will confuse it with the main body of the story.
Now let me break down some other things:
Dear Princess Celestia, As per your request I have made it to outer colony of Trottingham to investigate the World's Edge. To my surprise my friends, the other Elements of Harmony, (and me Spike!), joined me for the journey.
"And me, Spike?" Spike is presumably taking down a dictation, so he's only going to write down what Twilight tells him to. She would not tell him to write down "and me, Spike!" And I don't think Spike would take liberties. Keep in mind—we watch the show, we know that Spike does all of Twilight's writing, so the fact that he's there doesn't have to be explained—it can just be assumed.
Secondly, Celestia presumably knows who Twilight's friends are, and Twi calling them "the other Elements of Harmony" (a group title we've never heard her use in-show, mind) is kind of weird.
The letter as a whole comes off as forced, little more than an excuse to explain that "everyone is here" and give reasons. Again as I said earlier: not everything needs an in-depth explanation. The letter could easily have said "my friends all came with me for reasons of their own, " and you could have those reasons be gradually revealed as the story goes on.
The whole "you get the general gist right, but the soul is off" feel continues throughout the section you posted. You seem too worried about cramming in as much backstory and explanation early on as you can, and that causes your characterization to slip. What you should do, I think, is read your own draft, and try hard to imagine how it would play out as an episode of the show proper, and see if you can't spot parts that seem... incongruous.
Just keep in mind that not everything needs to be stated up-front. Concentrate on what's important
right now.