ouh, good question. to me it's the nicely mixed hybrids as well as true species with unusual colours (as long as they won't sparkle, though). something that hints at a possible BG story, but that doesn't seem to follow rules I could pinpoint. it's more like a guts decision, if that makes sense...
many adopts look just like they dropped into a shelf of paint. XD on the other hand it's a lot of work to create individuals for every other critter, so I understand creating and using templates.
rarely seen species might drag buyers in, I guess. I am partial to felines, but sometimes some critter you don't see every day on the main page tickles my interest more. again, it's a guts decision for me. :)

the third suggestion sounds a lot like easter egg adoptables: you provide an egg (or vial, or rock, or wingnut) with a set of colours and shapes, and the buyer gets to decide on species and gender, and if applicable, extra assets. :) this can create unique and beautiful creatures, you never know what people come up with on a whim. one artist I watch manages to hide links to the respective eggs on their announcing journal and one web page so people have something to hunt. :)

I can agree with Anbessa. I also see a lot of easter egg adoptables; a lot of really cool things can come out of those!

But mostly, it's just guts. XD