The GI preview mentioned story sequences being played out in form of comic book panels (ŕ la Sly), and everything seems pretty deeply connected with the storyline. The "sandbox" is in the openness of the world and how you interact with it, it doesn't sound like they are sacrificing story and direction at all. The fact that they are placing so much emphasis on bosses is another good indication of that.
I really like the mood they are going for. I'd go as far as to say that, judging from what little we have available, Sucker Punch seems to have cleared the jump from cartoony "mascot" game to adult "next-gen" with a greater margin than Insomniac (although R2 seems to remedy a lot of the R1 issues).