See, I started a thread at ENWorld not long ago about the concept, and it quickly got derailed into rather silly bickering. Then I got swamped with some things at work, and I didn't have time to get caught back up with the thread. Then I decided that I didn't really care to get caught up again anyway, since the thread had long ago ceased to be any kind of fun to talk about.
One point that I thought was worth noting, which seemed to be causing my "opposition" all kinds of grief, was that no game can be called a "sandbox game." Several posters kept saying that my discussion of sandbox in a theoretical sense was pointless and tried to point out specific games (OD&D game up more than once, but given the fixated obsession of the guy who brought it up, that was hardly surprising.)
The thing is; no game can be a sandbox. There's no such thing as a game that's a sandbox. "Sandbox" as a label is an artifact of play not of game design. While its true that certain products can encourage or facilitate a sandbox experience much more easily than others, a ruleset by itself certainly doesn't. It's a style choice if the person running the game. Two people running OD&D for two different groups could have a completely different experience with regard to how sandboxy the game is, depending on how the GMs run the game.
That said, I still think that sandbox as a term also loses any real utility when folks run around calling any game that's not a railroad a "sandbox." It's not. That just makes it... not a railroad. If the definition of sandbox is so inclusive as to be "not a railroad" then what's the point of using the label at all? Similarly, if its fans are trying to saddle the term with a bunch of qualifiers that are simply artifacts of a well-run game, then it also loses any utility as a term; it just becomes a synonym for a well-run game.
In fact, I've noticed this before in similar discussions; people who seem to be on complete opposite spectrums of this debate end up describing their games... and they sound very, very similar. A well-run game tends to have certain similarities, and trying to pass exclusive labels off as onto your games, when really all you're talking about is a well-run game, isn't a very useful pass-time.
Plus, as a pet peeve, "non linear" was already a perfectly viable term that was used quite a bit in the biz and by fans. "Sandbox" in comparison, sounds trendy, faddish, and juvenile.