Pluses:

1. Regarding the (formulaic) character designs, there is actually some form of variety when looking at two of the Dazzles. Those two aren't actually wearing any skirts and have other clothes on them to differentiate themselves from the others.
   
2. The song (both the lyrics and score) up to "Let's have a battle!" chorus was rather catchy and didn't contain a score that was rather bland and bare. Background vocalizations enhanced the sinister mood of the scene.
   
3. Most of the animation has improved. The walk cycle of the Dazzles look and feel much more organic compared to the previous movie, and the transitions are creative and/or seamlessly flowing from one cut to another.
   
4. As absolutely idiotic as Sunset's forced redemption was (and, by doing that, permanently ruining the reputation of the Elements of Harmony and massacring EQG's previously established slice-of-life narrative), continuity is reinforced by not retconning it.
   

Minuses:

1. At least two problems with the song:
   1. 
      
   2. It really hammers the sinister anthem of competition and makes the conflict feel very contrived. Couldn't Battle of the Bands be a little more subtle with its message?
      
   3. Its direction swerved once "BATTLE!" is chanted, turning from what was an interesting antithesis of Win the Crown to a very juvenile concept. (And considering its older base demographic and how much it'll rip off Monster High like last year [and likely Jem and the Holograms this time], you can make the concept of High School much more unique than that.)
      

[*]The green mind-controlling mist creates some confusion. Not the concept itself, but how it's spread. Can only the audience see it, or can the HuMane Five and Sunset Shimmer see it along with us?

Plus, seeing as Sunset was uncomfortable around the Dazzles, did she try to go to Principal Celestia's office and talk to her prior? We won't know if she did or not, but if she didn't, then that's the movie's first (if not second) pretty big plot hole.

P.S.: As for the trailer itself, the lampshade with Sunset turning into a she-demon was pretty funny and worked because… Well, let's face the facts; Sunset Demon looked really stupid.

Source: [Spoilers] Two New Previews for Rainbow Rocks on Yahoo