Things I look for in a helmet:

Comfortable fit on my giant Sasquatch cranium
Adequate ventilation
Light weight because I ride long-distance, too, and a heavy helmet takes a toll
Removable/washable padding so I don't get head-funk

I leave my mirror and headlamp on my helmet all the time. I have a Petzl Tikka lamp which I removed the elastic strap and zip-tied it through the vents of my helmet. It's a barely noticeable amount of weight, even after 100+ miles on the bike. The mirror is just a safety "always gotta have it" item for me, so it's a permanent fixture on my helmet.