The reason why Virtualbox and all these other guys (including OpenVZ and Linux-Vserver) are not on every desktop yet is because they all require reading reams of documentation to do even the simplest things.
Why can't a guest be a single file? HW configuration, disk images, etc, all in one large file. Why can't that file be stored anywhere on my filesystem and be opened the way I open all other files?
...$ qemu ~/vms/ie6.kvm
Or even -- gasp -- double click on it.
If I want to clone the guest? cp -r. When the clone boots, the VM notices if it's using a duplicate mac and tells how I can correct it. Why on earth should I have to dig around for virt-clone or equivalent?
If I want to migrate a guest from one machine to another? Shut down the guest, scp guest.img example.com:, fire up the guest on the new computer. There's no need for network daemons or NFS.
Delete a guest? Just rm it! If it's running then its blocks won't be recovered until the file is closed, just like every other file on your filesystem.
Dammit, if I want to run an old copy of Armor Alley, why can't I just scp a single file from a friend and double click on it? Why do I have to click my way through endless guis and google for obscure tools to do any of this?
Dear qemu/virtualbox/vmware/etc developers, FIRST make the simple stuff simple, THEN add wacky things like COW and live migration, OK? Your tools all feel like they were designed by overcaffeinated VMS dweebs looking for more job security.
Why must libvirt be such a thick shim with such horrible leaky abstractions? Why must it be so difficult to convert a VM from one system to another? Gimp, Photoshop and Inkscape can all share files, why can't KVM, Xen, and Virtualbox? (crazy external conversion procedures don't count of course) And why am I the only person who seems to care?
Today, virtualization on Linux is an absolute mess. If anyone has any explanations for why, I'd love to hear it. (this article hints at one reason of course, but I'm sure there's more to it than what repo stores the userspace code!)