I'm coming at this from a Windows user's perspective: To the best of my experience, RDP (aka Terminal Services) is a reasonably secure (barring mitm/server spoofing), efficient desktop sharing protocol with well-supported clients, that can be exposed to the internet when necessary without major fears of intrusion. To the best of my knowledge straight VNC is none of those things, which is where I get confused - why wouldn't a better desktop sharing technology be developed or used in the open-source world? I know VNC can be wrapped with SSH, but that seems beyond the reach of a casual user. X11 forwarding over SSH may be more or less efficient, I have no idea, but is definitely even more complicated, and doesn't (as far as I know) give you access to already-running stuff (no desktop sharing as such, just remote application running).