So i think part of what makes a super hero movie great is the dichotomy between your hero(es) and your villain(s).

I felt like the threat of an Ultron being virtually any and everywhere would create an inescapable feeling. Instead Ultron was more like Link from Zelda. Other than his infinite re-spawn capability this villain didn't scare me.

Now Scarlet witch on the other hand singlehandedly kept the avengers at bay. Where is a bigger more exploitative use of her evil side...that may have made for a better bad guy than Ultron.

To summarize, I never felt like our heroes were in mortal danger. The most difficult thing the Avengers had to face is evacuating civilians. Which for the record took about 25% of the movie.