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.
Not to mention Weiss' (White's) theme, "Mirror Mirror".
Blake's theme, "From Shadows", also qualifies.
Yang's theme, "I Burn". While mostly remixes of the previous three, the new song/lyrics at the end sound great.
Base Breaker: Adam's already developed a bad rep from the fans. Fans felt that Blake should've gotten more opportunities to shine, bemoaning how it was he who took out the Spider Tank instead. They may show interest in his role for the story and his connection with Blake, but even in this case, they're turned away due to his inexperienced voice acting.
Broken Base: Some of it is nitpicking, while others are more divisive.
The voice acting is easily the most contentious aspect of the series so far. The question is whether you're basing your complaints on the voice pitches or the voice acting.
The setting of the Black trailer. Most of the complaints come from the fact that it runs contrary to the more medieval/fairy tale-esque atmosphere presented in the first two trailers, though there's a chance that perception is sure to change once the series premiers.
Blake's theme. Like the setting, the complaints are mostly centered on how it differs from the two themes that came before it, rather than it being straight-up bad.
Ruby's Perky Goth voice at the end of the Yellow trailer led to mixed reactions as well. The high-pitched, somewhat childish tone does fit her, admittedly, given her short height, youthful appearance and apparentcheerfulness. However, it doesn't mean people particularly like it either.
Ensemble Darkhorse: Melanie and Miltia (the former being a bit more popular between the two) are shaping up to be this. The twins already have a good amount of fan art, and quite a few fans hope that they have a reoccurring role in the actual series.
Fan Dumb: The same day that the B trailer came out, the RWBY Facebook page was flooded with people asking for the Y trailer.
Fan Nickname: Fans have been calling Junior's weapon the "Batzooka", saying he must have stolen it from Batman.
In the same video, we have Deadb3ar.
Hate Dumb: While some complaints about the Black trailer are justified, there are fans who loathe it so much that they state the animation is inferior to the first two trailers. Needless to say, this is entirely baseless.