The a20 is just about capable enough for building your own laptop with - but at some point in the future there are going to be chips that will make quite a decent laptop... :D
From my *personal* point of view, here - for what its worth! are my perspectives
Price - the price point of the a20 is fair, but much higher and the price of all the kit you need goes above what a hobbyist can justify.
Open Source - Even if it can't be made at home, its still open source - especially with 64 bits you hit a point where the bus is just too wide to make home manufacture practical, on the other hand a FULL open source 3d stack is *vital*, given the increasing power, hardware decoding is less critical the CPU can handle it and you can just leave that bit unused if you have to *sigh* ...
SATA - less so for an embedded design or tablet, (although its nice) but for a laptop its *vital*
Display - cost effective LCD modules with a decent selection of physical sizes and resolutions including interface boards - this is often a big stumbling block in making a portable system... - keep doing what you are doing!
Free of any "active management" or "out of band" type remote control hardware