Finally saw it last night and loved it. I'm so glad I managed to avoid any spoilers over the past week. Juggling all the characters, storylines and locations of this movie was a massive undertaking and I think they did a wonderful job pulling it off. I couldn't think of a single suggestion I could offer that would make this movie better, and I'm the kind of annoying movie watcher who almost always has ideas on how a story could have been told better.
Regarding Hulk, my impression of his refusal to come out is that he's ashamed of his loss to Thanos.
Looking forward, I wonder if Thanos will continue as the villain in the next Avengers film or if he'll come to regret his decision to kill half the universe and play a role in undoing what he's done.
They gave his character more depth than expected. He ends the film the victor, yet several times makes a point of saying how anguished he is in his victory. Even after the snap of his finger, during that dream state, he says he lost "everything" to accomplish his goal. I'm just not sure I see him sitting on his random planet watching the sunrise over and over and waiting for the Avengers to come find him. Maybe he realizes what a huge mistake he made and how hollow his victory is without anyone to share it with.
Other things I'd expect in the next film... A reconciliation of Stark and Captain America, likely with a joint fight of them vs the primary antagonist where they sacrifice themselves to restore the universe. I'd wager the end of the next movie is a mirror of the end of this one. Everyone who died with the snap is restored with Stark and Rogers' sacrifice, and as the dead materialize, they discover and mourn the deaths of the friends who fought to bring them back. Many go into retirement (those contracts have to end at some point), Barnes becomes the new Captain America, and we get a tease of what the overarching threat of Phase 4 might be. (My personal hope is Sony let's Marvel have Norman Osborne back and he has an end-of-film cameo announcing the creation H.A.M.M.E.R., which he pitches as a much-needed replacement for SHIELD, but which will have sinister undertones that will slowly be revealed in Phase 4.)
Thanks and gig'em