My understanding is that we consider questions about running Ubuntu One in Windows to be on-topic for this site (thanks to Jorge Castro for clarifying this in chat).
But we close at least some questions about running official Ubuntu software (by which I mean, software developed by Canonical and part of the overall Ubuntu project) on non-Ubuntu systems as off-topic, like this question about Unity on distros other than Ubuntu.
Is the distinction just that Ubuntu One is officially supported on Windows, but Unity is not officially supported on any OS but Ubuntu? Or is there a more textured/complex approach that we take, in deciding whether or not a question about running Ubuntu (or Ubuntu-related) software on another OS is on-topic?
My motivation for posting this is this question about running Ubuntu Software Center in Linux Mint. This seems to me to be more similar to a question about running U1 on Windows than a question about Unity on other platforms, since like a Windows user running U1, a Linux Mint user may need to run the Software Center to access content through a service provided by Canonical. (The question is specifically about accessing proprietary payware in the Software Center, in Linux Mint 13.)
Actually, I think that question should be considered off-topic and closed, as the alternative seems to be for us to give preference to questions about obtaining and running proprietary software over questions about FOSS (even if that's not the effect we intend).1 I don't want to stand in the way of people using proprietary software, but I think free open source software should receive at least the level of community support that proprietary software receives. Furthermore, there are a number of similarities between Ubuntu and some of its unofficial derivatives like Mint, which might tempt us to consider some Mint questions on-topic...but we don't, and I don't see why we should start now.
With that said, I'd like to gather some feedback before voting to close that question (and others like it that might come around in the future).
1. To clarify, I am not saying the Ubuntu One client is proprietary software (it's not, it's FOSS), but rather than if AU allows questions about USC on non-Ubuntu distros because you need USC to buy proprietary software, but doesn't allow other similar questions about accessing other functionality, then we're giving preference to questions about proprietary software.