Have you ever used FreeBSD jails? I wish there was something as easy to use and flexible as that on Linux. The fact that each jail has minimal overhead and with dedupe on ZFS I can easily give every user their own "machine" while spending a lot less on hardware then a linux solution would require.
Having said that I also wish I could run FreeBSD on EC2 however I haven't put my money where my mouth is. I'm sure if you and many others wanted it badly enough you could sponsor development to fix any current issues. Also it would probably help if someone made a large business case to Amazon to ensure that things were targeted towards FreeBSD that would help too. (i.e. not needing an ext2 filesystem for the user provided kernel on micro instances, etc)
Absolute goodness of jails and the simplicity of making a small bootable readonly CDROM image made FreeBSD a clear winner for me in the late 1990's & early 2000's. In the 2010's... the world moves forward, now it's about having a standard virtual image.
The idea is very similar: you create a set of jails that behave as independent systems. Then every process started from a jail is tagged with a jail ID corresponding to the jail that the process was started from. All processes run in the same kernel and the jail ID gives the kernel just enough information to tell whether or not a process should have access to a given resource when sys calls are made.
I've never used OpenVZ so I don't know how they compare in practice but I can say that FreeBSD jails worked beautifully when I was using them heavily (about 2 years ago). They are super easy to manage and were as stable as you'd expect in FreeBSD (you'd have to be trying really hard to knock them down).
Having said that I also wish I could run FreeBSD on EC2 however I haven't put my money where my mouth is. I'm sure if you and many others wanted it badly enough you could sponsor development to fix any current issues. Also it would probably help if someone made a large business case to Amazon to ensure that things were targeted towards FreeBSD that would help too. (i.e. not needing an ext2 filesystem for the user provided kernel on micro instances, etc)