Check out what could have been as concept art for Ultron from Avengers 2 has hit the net from what looks to be an art book.
I do recall feeling a bit nervous about the look of Ultron when that first clip was released revealing Ultron talking, but I was pretty happy overall when I saw the movie.
I suppose some of these designs are a bit too menacing compared to the final look and what they were going for with Ultron in the film, which I thought was more of an adolescent "cry baby" (not in a bad way) more than an exterminator type of Ultron.
The third and fourth pics on the top row look pretty cool to me.
Updated: with more art of the Avengers.