Iron Man wasn't a "never heard of" character. Not even close. And Robert Downey Jr. isn't a "never heard of" actor.
Something like "Blade" is a never heard of character.
Batman Begins battling the effects of Batman and Robin, and being the 5/6th movie in said franchise while still being as successful as it was is a much bigger accomplishment. By far. It's like starting in the negative and pulling out all the way into the positive. With noticeable less enthusiasm for it at the start being as so many Batman movies had been released, bad year at the box office, etc.
Iron Man wasn't weighed down by anything. And it wasn't facing anything in its box office run till weeks later. It had all the chance in the world to be successful.
Batman Begins has ALOT of weight on it's shoulders and still did very well. And it's sequel? Without all that weight ... just look at what that did.
"So get back to me when you want to take things alittle more SERRRIOUSLY."