Don't get me wrong. I loved Banner/Hulk in the Avengers movie and FREAKEN LOVED Hulk in Avengers EMH. He brings a unique personality to the team and is very fun to watch interact with Cap, Iron Man Thor, etc. and I beleive Hulk is an INTEGRAL part of the team's origin but then you actually add in the Hulk mythos/universe into it.
Bruce Banner is classically "The Lonley Man" a wonderer trying to find a way to deal with the Hulk while being chased by the relentless General Ross as he keeps stumbling into situations only Marvel's (non-copywritten) version of Godzilla could handle alone. Hulk on the other hand is a force of rage and destruction desparately seeking a friend who won't hurt him while struggling with Banner's memories and Banner's own attempts to kill him when Savage. And when intelligent Hulk struggles with his own status as a hero and Marvel's version of Godzilla as well as his own cynical beliefs and attitude born of the abuse given to him in his Savage days.
With Hulk as an Avenger, his status on the team being VERY public where could Hulk's supporting cast come in without involving Red Hulk, A-Bomb or Red She-Hulk so early in the game? What if Hulk goes on a rampage? Is his status as an Avenger even put into question or his motives questioned by the authorities (sp?) or do they just shoot him. How much strain does Hulk's rather unstable character put on his membership on the team before Nicky boy says "Ok Dr. Banner, for public safty I'm asking to to be placed in this Guilded Cage in New Mexico until we need you." How would either Bruce or Hulk take that. When you factor in the themes, motifs, workings, characters and general workings of the Hulk Microverse within the large Marvel Universe context and Hulk being an Avenger for an extended length of time just makes less sense from the Hulk mythos POV.
This is just my personal thoughts and I do love Hulk on the team but... as I say for Hulk to be an Avenger, you have to give up at least 1/3 of the Hulk mythos. To be honest She-Hulk was a much better fit for the Avengers. However the thing that made Hulk work as an Avenger was his cynism and jackass-ish behavior that Jan is just too developed a character to really be altered to fit and not piss off hundreds of people. My own personal solution to the problem was to give the C-List Avenger Mantis an overhall to make her "Hulk without the baggage".
Thoughts? Solutions? Things I missed? People having a rather immature argument just to be immature? ...Anything?