The aliens in Avatar are aliens, they can look anyway they want them to look and we accepted it. Their flesh doesn't have to look like human flesh with pores, skin tone variables, hair, etc because they're an alien that doesn't really exist. The Hulk is a biologically human based creature with enhanced physical attributes ie, his muscles but his skin has to look like human skin, not alien, that's the difference.To me the aliens in Avatar didn't look like human flesh skin at all. They looked fleshy, but not as intricate as the skin variables of real human skin.
The cg in TIH sucked. The design of Hulk himself was fine, but the rendering was far inferior to the work ILM did. As i noted before, i think this was done on purpose, as i can't fathom anybody in charge over there looking at certain shots in that movie, and saying, "that looks great". I believe they wanted Hulk to look more cartoony in TIH to take away for the serious tone of the 2003 Ang movie, and make it more superhero-y.