The interesting thing is that lots of F2P games are becoming more profitable than the regular monthly subscription style games (WoW excluded, of course). For example, Lord of the Rings Online's profits exploded after they created a free-to-play model and converted their monthly system to a "VIP" system. It's tricky to do correctly, though. F2P games welcome hackers, gold farmers, and every other kind of unsavory internet type, while a lot of them are, in truth, "free to play, unless you want to ACTUALLY do anything". A good security and player management system combined with a system that gives paying players more fun or enjoyment, but not an overwhelming advantage, seem to be key.
As for Marvel... ehhhh... We'll see. DC Universe Online left a lot to be desired, and in general, I feel like MMORPGs are starting to hit the point of over-saturation. It's going to take something special to be a success, no matter how "free" it is.