So given the recent hubub about Sonic Adventure 3's domain registrations and Hunnid-P doing a new Knuckles theme, I'd like to pose this question: What are the defining aspects of a Sonic Adventure game?
I ask this because I saw someone arguing that Sonic 06 is effectively Sonic Adventure 3 (even though I'd give that title to Sonic Heroes). What's a legitimate reason for Sonic 06 or even Sonic Heroes not being a Sonic Adventure game, aside from the name? Are these differences enough to warrant a different series? Does it possess a distinct DNA, or did it lose its sense of separate identity in the wake of the other 3D Sonic games have come out since?