I went London-Shanghai (and ultimately Osaka) last year; one of the most affecting parts of that journey was going through the deserts of Kazakhstan and north-west China (e.g. https://www.cliché.net/photos/2012-04-02%20Trip/SDC16011.JPG and https://www.cliché.net/photos/2012-04-02%20Trip/SDC16084.JPG - hosted on my home connection so will be slow), which you miss by staying on the trans-siberian line. It's also a chance to see a really different side of China (as a European it's easy to imagine China will all be the same, being a single country, but of course it's as big and varied as, well, Europe) - and the route I took goes right past Baikonur, so there's always the chance of a rocket launch.

It's no good for going to North Korea (you finish in China, and as noted the line Beijing-Pyongyang is closed to tourists), but if you were just thinking of a big rail trip like the trans-siberian, have a look at the route via Kazakhstan too.