I am really excited to see Creature Types and Subtypes now expressed as Traits. This is a much more flexible system, and I feel that it can more accurately reflect the aspects of a monster. For example we can now have a Creature with the Aberration and Fiend Triats to reflect the creature is both an outsider from the lower planes and an aberration. Granted there were ways to get around this before with the extraplanar subtype but that always felt awkward to me.
One thing I don't like is having Hazard Traits and Creature Traits squished together. I hope when the final books come out they separate the two. It's a small thing but having a block of Hazard Traits and a block of Creature Traits would makes things easier to put together.