The villain felt underdeveloped and his reveal came over as very obvious and forced as well. If it slowed down and explored a little less areas it would have improved things. It did feel a little rushed at times and felt like it tried to do a little too much in the storytelling, as a result some ideas that were introduced like the science-is-good message gets lost. But it is still very good and one of their best easily since the Renaissance.
Big Hero 6 is not among the best that Disney has ever done and it isn't flawless.