I am deeply ambivalent. I love cs.se, but we haven't been able to attract the critical mass of OS experts required to make a vibrant community, and we only rarely get good OS questions. (Look at the operating-systems tag, of the 17 questions with a score of 5 or above, 16 were asked in 2012, and the one that was asked in 2013 is closed as off-topic.)
Every question that would be appropriate for the proposed os.se would be on-topic for one (or more) of:
There is no critical mass community of operating systems experts on any of those four sites. Partly this is because any specific OS question is likely to be off topic for at least one of those sites. Questions that include code or about the internals of a specific OS, would be unwelcome here. Questions about kernel internals would probably be off-topic in SU. Questions about OS in general, or about non-unixes (e.g., Windows, OSX, BeOS, VMS, z/OS, VxWorks, AmigaOS, ...) would be off-topic in U&L.
SO probably has the appropriate critical mass, but there the good OS questions get lost in the noise, good questions about OS and distributed system theory will probably get closed as off topic, and, well, SO is so large and diverse that it is difficult for any subcommunity to form and grow there. SO doesn't provide the technical facilities required to build a smaller subcommunity (e.g., portals for specific groups of tags, each with their own user rankings, moderators, community review-queues, chatrooms, meta, and norms about what makes a good/bad question.) SE doesn't provide the technical facilities required to allow a subcommunity to work across different sites (no cross posting or tag sharing.) The only option for building such a community is to propose a completely separate new site on Area51.
The problem (and the opportunity) are large. It's not just Operating Systems (although that's the only one that's close to achieving full commitment at the moment.) There are many, many, many, many, many, many, many, many, many, (oh, and also many, many, many, and many) Area51 proposals that overlap with CS.SE (machine vision overlaps with cs and stats, computer architecture overlaps with cs and electronics, compilers is another example that overlaps between cs and so, quantum-computing overlaps between cs and physics). Clearly there is demand for building cross communities that isn't being met in any other way than through Area 51 proposals.
I wish that cs.se could attract all the OS experts that are committing to the Area 51 OS proposal. (But we haven't). I wish that SE would acknowledge the need to allow cross posting and/or other ways of growing subcommunities. (But they won't). So a few weeks ago I sadly committed to the Area 51 OS proposal and, should it go to Beta, will make every effort to help it succeed.
Summary of the summary: I'm ambivalent.