Linux would benefit from a lot of things, most importantly getting rid of its acute case of NIH syndrome. FreeBSD's kqueue and jail subsystems are two good examples.
It took about seven years for Linux to finally get signalfd, relative to when kqueue debuted. dnotify and inotify!? Perhaps Linux's interfaces are marginally superior, but was it worth the wait and portability hassles? And kqueue still provides features unavailable in Linux.
The same analysis could apply to FreeBSD-style jails, except it looks like SELinux will be the final solution nobody uses. And the reason SELinux sucks is obvious; you can't truly separate policy from function, because they're indivisible in a security-conscious application, yet SELinux makes it very difficult to integrate policy into the development process.
This is why jail(2) and even chroot(2) are so powerful and so much more preferable than all the other convoluted solutions out there.