I think where Hulk may have difficulty capturing an audience, as compared with the other Marvel movies is this: we're talking about a 10 foot plus tall green behemoth. Not something you'd expect to see everyday.
The other Marvel movies, like Spiderman, X-Men, and X-2 have dealt with relatively normal people in terms of stature and appearance. The apparent humanity of these characters makes them much easier to grasp.
The Hulk is a far stretch from the above situation. Like I said, you're dealing with a giant, green monster. Suspending your disbelief will be difficult. But, also think about it this way: how fixated were you on the appearance of the Cave Troll in The Lord Of The Rings: The Fellowship Of The Ring? Or how about the Balrog? Were you really fixated on how good of a cgi character they were, or were you drawn into their respective roles in Fellowship? And you can also bring up Gollum, an impressive example of a digital character done well.
I'm gonna see the movie. I was a fan of the Hulk from when I was a little kid watching the show on CBS, and reading some of the comics, up through the animated series. I'm going to go into the movie with an open mind, and see what Ang Lee has in store for the moviegoer.