Here is what Ryan Morrison, the lawyer, said about the trademarks in his website:

"This is a TRADEMARK not a COPYRIGHT.

Those words are not interchangeable, so stop it! Trademarks are much more broad and powerful protection than copyright. They protect your brand and/or logos and slogans. They also protect against things 'confusingly similar,' so it’s possible you could be sued for naming your video 'Kids have emotions.'"

So apparently, trademarks are actually more broad and powerful than simple copyrights and can be used to shut down similar content, even if the title is different. This is where the danger alert starts to play. The biggest problem is not that they trademarked word combination itself, but the fact that they trademarked the goods & services of "teens react", "kids react" and "elders react". This would give them the power to shut down any competition if they wanted. Even if the court would not favor them, I don't think a lot of people would want to go to court against a multi-million dollar company. Anyway, I am not a lawyer, so I don't know how this would impact the YouTube community in reality. What I know is that the Fine Bros said they would abandon ALL "react" themed trademarks, but they didn't. This looks fishy... like a corporate take-over.

EDIT: forgot to add the Ryan guy website: http://ryanmorrisonlaw.com/attorneys-react-the-fine-bros-react-trademark/