1. Memory deduplication on kvm machines: matching memory pages are COWed. Hopefully there is a re-spin of the virtio drivers too (which rock, btw).
2. Way newer kvm/libvirt (the old ones sucked)
3. Python 2.6. They skipped 2.5 entirely, which is nice.
Just keep in mind, they'll be supporting it for a VERY long time, so RHEL6 will be looking pretty tired in another 3.5 years (the current age of RHEL5).
I'm learning to use virtualization and I picked Xen because seemed to me like solid technology - it's good for Amazon, Google and prgmr.com, so surely it will be good enough for me? On the other hand, it is not included in the kernel, its usage tends to lead to using anscient kernels, Red Hat and Ubuntu are moving away from it. I'm in doubt what virtualization technology should I be using.
(I am the author)
Edit: madvise, not mmap.
The most important feature is _SHIPPING_.