(spoilers)
This episode certainly had an interesting set of circumstances. When the three different ribbons to win were mentioned (fastest, most traditional, and most creative), it was immediately obvious which of the CMCs were expected to get each of them (Scootaloo, Apple Bloom, and Sweetie Belle respectively), and even the three fillies themselves acknowledged that assumption to each other. Yet, all three of them instead wanted to go for a different prize (Apple Bloom wanted to be fastest, Scootaloo wanted to get creative, and Sweetie Belle wanted to go traditional). This setup actually reminds me of the episode "The Show Stoppers" way back in Season 1.
But when Rainbow Dash, Applejack, and Rarity are invited by their respective younger sisters to provide the necessary help for building the carts, they immediately get carried away and pretty much end up completely stealing the show, going all out to win the naturally assumed ribbons, despite the fact that the three fillies all want different prizes. They even end up driving the carts, despite the rules stating that the younger ponies are supposed to drive.
In the end, all three carts end up crashing, taking out literally every other cart with them. So naturally, everyone gets angry at Rainbow Dash, Applejack, and Rarity, and the three apologize for getting carried away. They then suggest redoing the race, and agree to Cheerilee's new rule that the adult ponies helping with building the carts are not allowed to ride the carts at all (and we can see that ends up applying to every competitor, probably for the sake of fairness). In the end, the CMCs finally get real help from their sisters in building carts ideal for the prizes they want (though it should be noted that we don't find whether or not they actually win).
One interesting detail I saw is who were among the other competitors. There was Diamond Tiara with her butler, Snips with who I assume might be his dad, Dinky with Derpy, and of course a bunch of other background characters that I can't name.
There was also that song during the first run of the derby, and I can't help but get the feeling that it's referencing some classic cartoon about racing that I can't seem to remember what it was called.
(end spoilers)
