There are actually two issues with respect to this question, and I think it's important both are addressed if we're going to set precedent:
- Why this specific question was closed, and
- Oak's comment about it being premature to talk about Diablo III
To the first issue, I voted to close this question not because it's too early to be talking about Diablo III (although I agree that it is), but because open-ended comparison posts tend to have dubious quality:
- New compared to what, Diablo II?
- Why is that a valid comparison? Isn't it a sequel, not an updated version?
- What constitutes something worthy to be added to the list?
- Are we adding any value to the internet by essentially quoting the Diablo III marketing site?
- Where does this list end? Do we update it even after launch with patches?
- If we don't, what value does this list have after Diablo III launches?
Other sites on the network field this type of question on occasion, and it very rarely winds up being a constructive use of the engine: people have different expectations, it invites one-line answers, the answer never becomes canonical, and we're just left with a mediocre question and answer "pair".
And we've shown for other high profile sequels like Portal 2 and Witcher 2 we can avoid these "soft" questions and do well with just questions people are actually facing in-game.
Which leads to the second issue, and more to the specific point you're asking about: when is it okay to start asking questions about a game? I think the answer is pretty simple: when people are reasonably expected to have questions about playing the game. That would include launch as well as any beta, or even alpha (c.f. Minecraft before December 2010). The baseline is that a member of the public has be able to play the game, not speculate about it or rehash announcements about it.
To both issues, Stack Exchange is expected to solve problems people actually face, not provide a launching point for discussion about a topic, especially on a game that nobody can actually play yet.
The problem gamers actually face is having trouble playing a game, and without being able to have first-hand knowledge of the game, nobody on this site is in a position to provide expert answers about the topic. Let Blizzard and the rumor sites own pre-release information: we'll rule the post-release help and answers arena.