The problem is that the JLA is DC's best, while the Avengers don't represent the whole of the Marvel U. A true Marvel All-Star team would have Spidey, the Hulk, various mutants, etc.
Thor vs. Superman - Hey, Thor's a god, and Clark ain't. I think this would be a hell of a fight. Superman is vunerable to magic, right? Would Thor's lightning be considered magic?
Cap. America vs. Batman - Again a great fight. Is it strickly hand-to-hand, or do they have their shield and utility belt? I remember one Batman writer once saying that in this fight "Batman would always win, because Batman cheats." LOL!
Iron Man vs. Green Latern - I love the George Perez poster everyone is refering to, but this match-up seems unfair. Everyone complains that Superman is too powerful, BIMO, Green Latern is the one that seems too god-like. Couldn't he just take apart Iron Man's armor with a thought? Don't know much about Quasar. Maybe he could take him.
In the end, I've seen these vs. threads go on forever. A writer could make anybody beat anybody depending on how he writed it. I wouldn't want to be the guy writing JLA/Avengers. (Busiek?) Talk about pressure!
Ms. Teschmacher: "But Lex, my mother lives in Hackensack!"
Lex glances down at his watch, then back up, and shakes his head.