In other words, if it looks cool, and shows-off DKPSystem in a way that makes other guilds (non-customers) want to move their sites to DKPSystem, then he'll likely add it.
Bingo. This is the defining characteristic. But it is all at my fascist discretion.
So far, every Featured Guild ever has used the Advanced Layout Options. With the new "Edit CSS" option that doesn't require the Advanced Layout Options, it is theoretically possible that someone can do a total marvel of CSS and make their site "Featured Guild" worthy without using the Advanced Layout, but as of yet, it's not happened, and admittedly, would require some pretty amazing modifications.
Ultimately, I have to be impressed, and every featured guild so far as impressed me, some of which (like the Envy Site) made me go "dayum" for technical reasons. If you bust out some sweet javascript, I'm usually very impressed by that, for sure. Some awesome javascript sites would be Renewal (for the dynamic menu), Envy (for the countdown parser), and Apocalypse (being the first site with the option to dynamically switch between left and right orientations of the menus).
Impress me, and you're a shoe-in on the Featured Guilds.