Who cares? (Or, Why not share and win?)
The question I've never seen answered about OpenSolaris is this: Who cares?
Sun and open source are uneasy companions. There are a few features present in OpenSolaris that aren't present in Linux (or *BSD, for that matter), but these features aren't sufficient to overcome concerns about the sustainability of the OpenSolaris effort, considering the small size of the community and the strong control exerted by Sun.
A vibrant community is essential to the commercial acceptance of an open source software solution, because it ensures competition between support providers; aids in expansion of hardware support, software compatibility, and testing; and bodes well for long-term support.
It would be better for Sun to license technologies such as ZFS in such a way that they could be incorporated into other open source kernels, and leverage the large and diverse developer and support communities around those kernels.