I like wide shoulders. My near-death experiences have come on rides with no shoulders and drivers who seemed to think that an inch or two was plenty of room to give me. Another terrifying moment came on a road with no shoulders and blind corners. A logging truck waited for a suitable place to pass, then got impatient and passed in an unsuitable place, a car appeared in the oncoming lane, and the truck pulled back into my lane, forcing me into the ditch (thank goodness there was a ditch and not a rock wall). I bounced along, inches from the trucks wheels, stayed upright, but broke two spokes. The truck sped off (of course.)
I don't mind a busy road if there are wide shoulders. If there are no shoulders then a lightly-traveled road is crucial. Also crucial is a rear-view mirror. Check for passing vehicles. Then check the oncoming lane and see if there will be room for the person coming from behind to swing wide. If there isn't, pull off the road and let him/her pass. It's a hassle to have to pull off and stop, but you'll (hopefully) stay alive.
I also like roads with smooth surfaces, roads through beautiful and interesting scenery (trees are beautiful, but I've ridden roads in the northwest where there were two walls of trees on both sides of the road for mile after mile. That gets boring really fast.), roads with places to go to the bathroom and get water (interesting juxtaposition, huh?), lots of campsites, etc. A road with a tailwind is especially nice, as is one with gradual climbs and more downhill than uphill.