I think Dr Strange is probably the riskiest movie for Marvel in Phase 3. However, a lot of the ideas about why it will not be liked are the same as the fears surrounding several previous Marvel films, most especially Thor. I remember it being derided and doubted because it had magic and "corny" mystical elements to it as well, but in the end it was well received because of great casting choices and by being a solid, enjoyable movie. Dr Strange looks to be turning out the same so far, which is to be expected given Marvel's track record.
I think the larger -- and more likely -- fear is that superhero movies in general might begin to feel repetitive and uninteresting. That "new" and "wow" factor has definitely worn off, so people should question how long this current trend of popularity will continue, especially with so many studios trying to get a piece of the action. Over-saturation of the market is definitely happening at the present time.