I've seen many people say that the songs were lame and forgettable, and while I can agree many of them aren't as good as anything I've heard in FiM, 'Helping Twilight Win the Crown' was actually pretty damn cool. It was complimented by some great animation and having the humanised Mane 6 dance certainly wasn't as cringe-worthy as expected. Quite the opposite.
Giving Spike Dog the ability to speak was another plus point as I had hoped. By the end I thought his new temporary design was spot on, although him and Rarity getting as close as they did at one point was... interesting.
I already knew Sunset Shimmer had a really nice character design, and her Sunset Satan transformation was pure FiM (very reminiscent Chrysalis' reveal). Her intent along with the execution of her plan was kind of lack-luster, and the end felt like a slightly more extreme "Trixie redemption" + "NMM defeat".
As for Flash Sen- Uh, Brad, he didn't feature as much as I expected. Even when he was more prominent during the second half of the movie, he didn't get much screen-time. Twilight suddenly falling for a guy seemed rather pointless other than the fact that he happened to be Shimmer's ex. Their dancing however... made me forget all about that for a moment. They also hinted at the fact that when Twilight returned and bumped into his Equestian counterpart, they might go somewhere with it. But we already heard that the writers aren't planning on going down that round in season 4. In the end the whole "Brad" affair is something I'm entirely indifferent towards.
And for the final spoiler,
best part of the movie right here.