I love this episode. If there's a point where Friendship is Magic evolves into the show I love so much, this is definitely it. The writing is at its peak, the animation is amazing, the characterization is great... It's easily one of the high points of the first season.
Given that the episode is largely based around Applejack and Rainbow Dash, this means that Ashleigh Ball will be talking to herself a lot. What's impressive about that is that it never sounds like she's talking to herself. Sure, it may be largely the work of the voice director that makes the two characters sound so distinct from each other, but it still takes some talent to pull off. Indeed, the great chemistry the two characters have with each other is partially due to her voice work.
This chemistry is part of what makes the humour work. The friendly interactions between the two ponies are very entertaining, and even when things get less friendly between the two, it never gets to the level where it's obnoxious to watch, and that's partially because it tends to manifest itself as action. Speaking of which, the action is itself highly entertaining, being fast-paced and well-made. A rising level of excitement makes up a large part of the episode's entertainment value.
That's not to say that it doesn't have anything else on offer, however. In particular, dialogue is very well-written. While it's filled with puns (leading me to suspect that MLP's characters are very fond of them), so many of the lines are entertaining and quotable, and just plain well-written. There is no shortage of great jokes and funny moments.
This is just a great episode. While "Winter Wrap Up" was the transition into more confident writing, "Fall Weather Friends" is the confirmation that yes, this quality is here to stay. This is where the show truly becomes the one I love so much, and it's easily one of the season's best.