The tone is mostly dystopian. Nobody lived happily ever after. If anything, everyone lost more than they gained after Endor. Pretty much, ROTJ will become anti climatic. The Jedi rose only to be crushed (again), Luke went into exile, the skywalker lineage is again tainted by the dark side.
EDIT:
Killing off Han Solo isn't what is going to ruin the film (for me), because it makes sense. Harrison is old, likely to die before the trilogy is finished, his character's death at the hand of the villain is fine for bringing a sense of darkness and danger to the film, and will pull on the emotional strings of the audience. Maybe keeping Luke away from Han will make it even more tragic, as it gives him a push in the right direction to step out of his exile and take action. Who knows, if properly executed, the film will work, but at a first glance, it sounds dissapointing.
Nothing to see here, move along.