Making any sequel is difficult.  So often, it seems like the people with the least idea about what made a film work are the people who actually made it.  Audiences connect with films for all sorts of reasons, and trying to reproduce that lightning-in-a-bottle can end up an exercise in frustration.

If you were to ask me, though, I think "Iron Man 2" has a shot at being one of those cases where they get everything right and actually improve upon the well-liked original.

Why?

Mickey Rourke.

Why else?

Sam Rockwell.

One of the main reasons the original film worked was the unconventional decision to cast Robert Downey Jr. as Tony Stark.  Casting is the thing that becomes most crucial when putting a film like this together because, in order for the mainstream to accept the more outlandish elements and ideas of a superhero film (or a SF or horror or other genre picture), you have to cast people they can really identify with to help ground things.  The audience believes in Iron Man because Robert Downey Jr. believes in Iron Man.  And when you cast someone as strong as Rourke, right in the middle of his comeback moment, opposite Downey Jr., the results could be spectacular.

And Rockwell is a hard guy to pin down for studio films.  He's practically allergic to them.  Casting him as the other major villain in "Iron Man 2" would mean you'd have two great actors, with major-league charisma, both going up against Downey Jr. this time.

I don't know what they're playing yet (the trades seem to be arguing about what characters they're attached to, but they're getting the backgrounds of the characters wrong as they argue), but it doesn't really matter.  What's exciting to me is that this is the sort of cast you put together for a serious Oscar-bait end of the year movie, not a summer superhero film.  That's a very good sign.