These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
There's a little something for everypony here! Twilight's theme is fast-paced, mostly orchestral (with a little light rock) number, Rarity's theme is a mellow dream-trance style song, Applejack's theme is a very catchy folk-metal/southern rock song, Pinkie's theme is a hyperactive progressive trance/dance song, and Rainbow's theme is pure rock.
The current status of the Spiritual Successor. Either people are denouncing it, saying that it'll never be as good as the original with ponies, or they're optimistic that it'll be good regardless of what species the characters are.
Rainbow Dash was considered this when she was revealed at Canterlot Gardens. Safe, hard-hitting normals, great pressure, a really fast dive kick, excellent mobility, and the only things she's lacking are health and supers. Granted, she's obviously incomplete, so it isn't that big of an issue.
Memetic Badass: If some of the comments in one of the streams is to be believed, Gummy.
Memetic Mutation: One guy on the EVO stream attempted to type #Swag as is a typical stream thing to do. Unfortunately for him, he made a typo, and it became #Swaf instead. Cue all the stream monsters spamming Swaf and the birth of an inside meme.
Scrappy Mechanic: In the Pon-E3 stream, one of the developers mentioned that characters gain extra armor when their health is low, and that this feature may be this to some players.
They Changed It, Now It Sucks: The news that it'll become unrelated to Friendship Is Magic kinda deflated some hype for those who really wanted to duke it out with their favorite ponies, but the fact that Lauren Faust is joining the project has softened the blow. Somewhat.