Some questions about this software have been closed as it is not released,
However the OS has been released to developers. This means there is a chance we can have questions answered by anyone with the system.
There are a few reasons why questions about pre-release software is off-topic for Ask Different:
A lot of companies, most notably Apple, require everyone with access to pre-release software to sign an NDA. Anyone that asks or answers questions about software covered under this NDA are in violation of its terms, and I don't think having open questions on the site tempting users to break their NDA is a very fair thing for Ask Different to do.
Pre-release software often isn't even complete. This means that the answers to the questions can change before the software is even released. Granted, answers to questions about released software do sometimes change over time, but not with the volatility of pre-release software.
Pre-release software often has many bugs that will get fixed before its released. This means that questions about application behavior or finding workarounds will be obsolete by the time the software is released.
It's possible that there are questions about pre-release software that aren't affected by any of these rules, but most are by at least one and it's easier to have a blanket policy against such questions than to have to decide for each one.
With regard to the questions about Mountain Lion, #1 definitely applies, as well as #3. #2 may not apply to announced features, but they could still conceivably change before it's released this summer. (I remember when Apple announced ZFS for Mac OS X Server and it went nowhere)
If you have legitimate access to install and run Mountain Lion, then by definition you have a dedicated support area that is highly targeted towards discussion of it without the sorts of arbitrary limits of scope that we (rightly) have here.
Anyone who has a genuine question about ML that is more than curiosity for the purposes of being a Beta Hero at home is either ignoring the most suitable resource which they have paid cold hard cash for, or has managed to grab a copy by other means.
It's all about knowing your audience. Anyone can buy an iPod, iPods are on topic for community crowd sourced support on an unofficial forum like this. Likewise, anyone can join the developer program, but that doesn't mean the same rules apply. The audience is markedly different. Allowing such Beta questions restricts said questions to a minority portion of the audience, which means that only a tiny proportion of users can have valid input or view for the purpose of answering or even voting, and worse still at the risk of alienating that audience that sees this place as an accessible place to look for help and doesn't want to see a whole load of minimally useful stuff that they don't understand or care about which is likely to become outdated or inappropriate very quickly floating to the top.
I am a member of the developer program, and I wouldn't dream of asking any questions I have about it anywhere other than the official place. I've paid for that support, I have not only a right to use it, but an expectation to see anything I do ask answered appropriately. I don't have the same expectation of a free site on the internet, even a quality one like here.
Whenever deciding whether Ask Different should allow or close:
If Apple provides a public area for discussion of an Apple product, then Ask Different should not close or delete questions about that product.
Example: for Messages, Apple provided a discussion area long before the release of OS X 10.8 with Messages.
Mac Developer Program members should use Apple Developer Forums and other areas designated by Apple.
Along with Kyle's points about Ask Different policies, I favor having Mountain Lion pre-release issues reported directly to Apple rather than discussed here, because this ultimately helps Apple improve the software before it is released.