We don't EXACTLY know what the last stage (space) is all about, but when you can sorta do whatever you want to whatever planets out there, i would sorta call it a sandbox.
Ofcourse, what Idiota might've meant (and what was apparently terminologically nazified ("Hey! That'd make an awesome song title!" -Nathan Explosion)) is that it's free-ended to the point that you don't even have short-term goals/quests. You just advance through levels. Heck, if you don't wanna, you can probably even not do that.
Another chance, ofcourse, is that EA is slowly getting it's claws on Spore, caring for how it sells instead of the idea. Luckily enough, i am yet to see any question sheets a-la "What do you think of feature x in Spore", which is a rather sure indication that EA is taking control.
PS. On that last note, *shudder*