The scripts for the Iron Man films are slapdash messes, and depend largely on Downey's remarkable talent and charisma for any shred of humanity - subtract him and there's not much to draw people in. There's a lot more going on in terms of recognizable human relationships and interaction in Green Lantern than in most superhero films. Thor is completely vapid in that regard, though Captain America looks promising.
Though I suppose a large percentage of the audience for these things is more interested in debating how far someone can throw a magic hammer than in why anyone would care.
Though the Iron Man movies did rely heavily on RDJ, I think the first movie's plot worked.
I didn't say anything about the plot. I suppose it worked; the plots to most movies do. Plot is also the least interesting aspect of film for most moviegoers and in most cases is the element that has the least bearing on a film's success.