Why ot usual noutbook?)
Mostly the size difference - It's closer to a nintendo DS xl than a notebook. It (should) also have a significantly better battery life than pretty much anything else at that price/size, 5+ hours while actually being used to play games (I hope). It also has "proper" game controls - D pad, analog sticks and shoulder buttons.
At this point it's no longer really 'cutting edge' tech, it's a couple of generations behind mobile phone SoCs, but it should have more than enough power to deal with emulators and oldschool (or even a couple-of-generations-old games) on the run, as well as games aimed at mobiles.
Plus it's a 'full' linux system, so you can still run firefox, openoffice, develop in vim or and similar if you really want. Having a full keyboard means it's not insane, and the previous version (the pandora) actually wasn't awful to use, which is amazing considering the size. While I haven't actually held a pyra yet, I assume it'll at least be no worse.
But it's a small-run niche product. And that comes with a price premium.