These are the things that I look for before I join a site:
Is it a pet sim or a pet site? I'd much rather play a sim game than a pet site.
A streamline and fun explore system. I love it when I can find more pets/mates, items, and stats etc. in explore. Lioden is probably the best known example of this type of system. I also really like Wild Horse Valley's explore system as it's building itself up as more of a rpg system.
A genetics based breeding system. Not a huge fan of color wheels and color mixers in general.
Nice art and varied markings. I've never been one of those people that can just look at stats and go based on that. It just doesn't hold my attention.
Above all, I just want a sim that isn't about wolves, dogs, or horses. The fact that lioden was about lions was the main reason I first got into it once upon a time.
I will definitely stay updated on this dragon sim, simply because it is a dragon sim.