It's easy to forget that, before it became the franchise model for everything, "The Avengers" didn't sound like a great idea.Yet Marvel and Joss Whedon took a pile of potential excess and smoothly honed it into a burst of pop entertainment. And amazingly, they've managed to make it all work yet again with "The Avengers: Age of Ultron," assembling a ton of moving parts into another thoroughly fun - if not quite as effortlessly harmonious - blockbuster.