My god. It's ages since I got excited about anything in the OS (or OE) world. It's got to have been a long time since I had anything nice to say about Sun but here I am excited about Solaris 10.
Here's why: Zones. (For Sun's official site, read this link).
Complete virtual computers (including root access) that run inside the global (real) computer. Whoa! Read that sentence again!
Of course, we never may get to use this feature but it seems like a pretty cool idea. Run lots of separate customers on one big box. (Although, running one big box can cause it's own problems: big Sun machines can take an age to reboot; can't share apache processes across the zones (without running it in the global zone and thus breaking the security/separation provided by zoning) so memory use is up and the machine is less efficient).
It sounds a good idea to me.