Ultron sounds awesome, based on the Comic-Con panel footage. Between his role in the film, and the announced graphic novel from Rick Remender and Jerome Opena - The Rage of Ultron - which is already being hyped to do for Ultron what The Killing Joke did for The Joker, and Ultron seems set to get a major boost in prominence. He's already regarded as one of the heaviest hitters on the Avengers rogues gallery, but this seems likely to propel him to the A-list of iconic Marvel supervillains.
This is something really canny that Marvel has been doing, across both the films and the comics. People have been talking about them elevating the heroes they own the movie rights to, but the same has been happening with the villains. For the longest time, the holy trinity of Marvel's definitive villains has been considered to be Magneto, Dr. Doom and (depending on what era of Spider-Man we were in) either Doctor Octopus, Green Goblin or Venom. None of whom can be used in Marvel Studios films. But through a combination of their film portrayals and prominent featuring in comics storylines, Marvel has been steadily elevating a new wave of villains to be seen as part of that A-list, including Thanos, Loki, Red Skull, and now it seems, Ultron.