SPECULATION (only a spoiler if I'm right about any of it!):
There have been hints from Feige and Whedon that this film will be dark and to "bring tissue" sending rumours of death to and fro. So if true, here are the most logical casualties to my line of thinking:
1 - War Machine. Don Cheadle is in the scene where they try to lift Thor's hammer. I've not seen him in any shots from later in the film, including the new trailer where they all stand back to back fighting the oncoming machines. He has been a fixture in all of the Iron Man films and thus would be a significant loss.
2 - Jane. While she hasn't appeared in any trailers nor is she listed in the supposedly official cast list on IMDB, she is something of a main character in the MCU and her death would be a blow to Thor causing grief to all females who adore him.
3 - Pepper. Like Jane, this is way speculative since she hasn't appeared in any trailer sequences nor is listed in the cast list, but again a semi-main character who would deal a crushing blow to Tony (and maybe send him over the edge into the side he will take for the coming Civil War of Captain America 3).
4 - Dr. Selvig. Again, not in the trailers, but Skarsgard is listed in the official cast list so he is in the film and has been a mainstay to the story line thus far.
5 - Maria Hill. Cobie Smulders is on the cast list and appears in that first scene. Owing to her complete lack of any super-power, it's easy to speculate she might be a casualty of that first encounter.
I seriously doubt any of the main six or Fury will be the one (ones) killed by Ultron (assuming a main-ish character really does die at his hand). I also don't think it will be Wanda or Quicksilver since they are just being added to the Universe and thus haven't had time to be endeared to audiences (thus, why the need for tissue). The most likely victim, to me, is Rhodie.