I Dream of Pink

http://www.fimfiction.net/story/50330/i-dream-of-pink

Just a disclaimer, I've not read the sequel to this yet, though I do intend to. It may be that a great deal of what I say doesn't apply, however I'll just put it down and let you decide that for yourself.
Before I get into any of these critiques, I'd like to say that the world you've created is compelling, interesting and engaging. This is mostly because of the great set up you've got with Rarity and the events leading up to present. The story is cute, the narrative is fun and easy to follow and I found it overall to be incredibly enjoyable. Please don't take the critiques following as detracting from this, its a nice piece and I found it very enjoyable to read.
All right. To begin, there's a litany of very minor grammatical errors that I honestly had not noticed until my second read through. I'll list a few examples of common errors , however a good proofreader with a fine tooth comb would be needed. Like I said, the scanning on my first read didn't even notice these, but as I look more closely I see more and more.
"Almost half way done"; 'mind numbing nature of this' Should be halfway and mind-numbing.
There's actually quite a few instances of this where there's a missing hyphen or separated words that should be compound. A spell checker obviously wouldn't pick up on this, so the only option is to literally read through and find them all.
'The dull mind numbing nature of this was wearing on her.' should be dull, mind-numbing as it is missing the comma.
Likewise ' At the start she had actually enjoyed being" should be At the start, she.
You've used a lot of appositives and introductory clauses to make and emphasize points, so the missing commas become more important as the clauses pile up. Ramming two clauses together without a comma leaves the brain without a clear place to stop. It's less important here but it also appears elsewhere
This leads to sentences like this 'When Pinkie would throw a party she would make a special cupcake for Rarity, usually with a sugar gem on top or even one of Rarity's dress designs on top, drawn with frosting.' There should be a comma after 'throw a party' which means there are 4 separate ideas/clauses occurring here. It's not quite a run on sentence and not grammatically incorrect, but having that many ideas in one sentence feels disjointed and all over the place, it may be something to keep an eye on.
There are other minor issues ("I'm glad you like it darling" is missing a period. "hehe, ok Rarity, I guess the Cakes" is not capitalized at start. "More than one of Rarity's hoofmade outfits was" should be were as you're talking about plural. "not that she had anything about pink" should be against pink) but none of these really get in the way of the reading of it, and I'd like to talk a little about the flow here.
Something was bothering me on my first read through of this, and it took quite a while for me to put my finger on it. The flashback/memory/straight exposition section of Rarity recalling the relationship up to now ends up being too telling rather than showing because we're simply being given an account of what happened in terms of events. Fortunately, there are tidbits of Rarity's reactions to what happened interspersed throughout to keep us from feeling like the author is just trying to shove information at us to get us up to speed. Even still, it feels very much like a "Here's a run down of what's happened so the present makes sense" kind of section.
The one exception to this was the description of the first date. That is the feel that the description of the parties, being asked to go out, and everything preceding and following should have had. In the description of the first date we're given Rarity's view on specific things that happened. The way they were dressed, the expectations, how Pinkie could hardly keep herself still. We need more of that type of exploration to make the rest of the exposition feel like we're seeing the story, not being told "this happened, then this happened, and now you know how we got together."
One thing that you might wish to explore is putting in dialogue into the memories. I say this because the dialogue at the end sounded very nice and very in character. I feel kinda lost thinking about how Pinkie Pie would have asked Rarity out, and what she would say. Since you have both their voices down so well, I think that would be something that could really benefit from some speech. Again, what's needed is something that isn't generalized. A conversation, a set of specific circumstances surrounding these events, all of this would help the impact of that exposition. Right now it feels like an author talking at its readers, rather than a viewer looking into a character's memories.
I hope I didn't turn you off too hard with the grammar issues at start, and I also hope that my suggestions are clear enough to be understood. And, as always, feel free to disagree with my opinions, as they are simply my opinions. I do hope you'll at least taken them under consideration however, because I do think they merit that.
Now, on to read the sequel. Err..... ::checks the time:: perhaps tomorrow.
Best of luck.
-Duskrider