I'm not particularly fond of the way React mixes presentation (returning nodes in render()) and logic. Particularly things like:
var classes = _.reduce(["flipped","correct","wrong"],function(m,c){return m+(this.state[c]?c+" ":"");},"",this);
seem very inelegant compared to say, Angular's ngClass, which does the same thing in a data-driven way. But, that's not to say Angular is free from template logic either.