Oh boy, you're going to get a lot of different opinions here. Many of them, like mine below, are little more than preferences based on sailing styles and circumstances. There are so many elements that make a good bluewater boat, but SailingDog's and Kwaltersmi's lists are a good starting point..
Among other things, I like wide, clear sidedecks, uncluttered by deck-piercing shrouds and stanchions. I like the security of raised bulwarks and tall 30 inch lifelines. I like the chainplates horizontally through-bolted into the hull, and stanchions similarly horizontally bolted to the raised bulwarks (both for strength and resistance to water intrusion). Also, there should be space for a junk-filled milk crate on the coachroof (just kidding on that one):
(Pacific Seacraft Crealock 31)
I also like a protected rudder, preferably with the propeller in an aperture:
Good anchoring systems are important too. I like the ability to store and launch several anchors from the bow and stern. Okay, maybe not this many, but you get the idea:
I'm sure others will add their wish lists. This should be an interesting thread...