I'd like Mac OS X but on a Lenovo T or X. That would just be perfect (assuming it ran as well as it does on a MacBook).
Lots of niggly stupid issues - like terrible audio quality, or poor LCDs, or poor build quality requiring stuff like shims under keyboards to stop them rattling (seriously?!). Crappy touchpads. Dumb experiments with the keyboard and touchpad/trackpoint buttons.
Every time I think I should give one of the newer X's a proper go, I go and have a play with them and realise that they're still full of compromises.