Patrick Stewart have said before he had no intention reprising his Picard role (can't say I blame him, Nemesis was probably the worst of the NG films). So the fact that he has now changed his mind must mean the script or story must be up to a certain quality standard for him to agree to return to the role.
Of course, the producers could also have offered a nice fat package filled with lots of $$$ to him, but at his age, I don't think Stewart really needs the money. I have heard Stewart does have a large say about the storyline, so that would likely be the stronger incentive to return to the role.