Well DC has 3 different JL's at this point in time, there has also been various incarnations of the JSA and also Teen Titans, Red Hood and the Outlaws in new continuity as well.. Those are the ones off the top of my head.
I think one of the things for DC is they get so much attention on the main Justice League, some of the others aren't known about - even if they are good. Justice League Dark for example is actually pretty cool if you're into the more magical based stuff. Red Hood & the Outlaws has been very good IMO (from what I've read so far anyway).
I think Marvel focuses a lot more on team ups than individual characters. I'd say because historically that's where they've found the most success. For example, Avengers is made up of really only 1 or 2 A list characters popularity wise, but stick them with a bunch of B-listers and suddenly it's a hit (I'm not referring to the movie popularity BTW). X-Men and Fantastic Four - only Wolverine has ever really stood out as a stand-alone character - none of the other members of those teams could IMO be solo characters. With the exception of Spider-Man I would suggest X-Men and Avengers are more popular than any of the stand alone characters especially those that are involved in said teams. I could be wrong - but from my observations, that's how it seems. DC however have Batman and Superman as strong stand alone titles, as well as the various Green Lantern issues and WW's solo title seems to have historically been strong too (maybe not in recent years, but back in the early days). Flash is also quite strong as a stand-alone title and character, having been popular without needing to be included in the JL. So basically Flash, Superman, Batman, Green Lantern and WW are all A-list characters in a team with 1 or 2 B-listers (Aquaman, Martian Manhunter/Cyborg), where as Avengers is Captain America and Hulk (the A-listers) with Thor, Ant-Man, Iron Man, Black Panther, Hawkeye and Wasp (the B-listers). Thor was maybe an A-list hero popularity wise, but with the exception of Marvel fanboys, he's never seemed overly popular to me until recent years thanks to the movie and gaining popularity of the Avengers themselves.
I'm not as familiar with the DC Universe, but most of the teams (or at least the main titles/well-known teams) are based off the Justice League or Titans.
Marvel has both more and complex teams... Of course you have The Avengers (New, Young, Mighty, Secret, etc), Fantastic Four (and Future Foundation), X-Men (Force, Factor, New Mutants), The Defenders, Heroes for Hire, Alpha Flight, Guardians of the Galaxy... The list goes on.
@derf_jenkins: Iron Man has always been a B-lister until RDJ. Even then, he's still not as popular as Spider-Man, Superman or Batman.
Traditionally I'd doubt that Iron Man would even be in the top ten most popular heroes...I'm sure he is now due to RDJ and the MCU.