For many years Myanmar was the sole country on the planet you couldn't drive your own vehicle into.. preventing a trans-europe->se asia trip without shipping your vehicle.
That has all changed in the last 12-24 months, and multiple groups have reported it's now possible.
I live on the Thai border with Burma, and while it's possible it's still tricky. The road from the Thai border to Yangon has one lane total (!) so it alternates directions depending on whether it's an even or odd day.
China won't let you into the Tibetan Autonomous Region except as part of an official tour party from within China. You can go around further to the north (I went that way by rail a few years ago), but most people on such a road trip would like to see India (and probably also Nepal), and to then have to go back all the way to possibly cross the border in disputed Kashmir or realistically to Uzbekistan is a bit of a detour to say the least.
(Central Asia is fascinating and I do recommend the trip through Uzbekistan and Kazakhstan at some point).
That has all changed in the last 12-24 months, and multiple groups have reported it's now possible.
http://wikioverland.org/Myanmar
(The details are still a little short, but we're getting there)
I'm planning my trip now :)