What qualities should I be looking for?
Bigger is better.
Heavier is better.
Fire ratings get better as price goes up, but water damage is the big risk in a fire.
Outside hinges provide better access to the safe interior than do inside hinges. Some will argue that inside hinges are more secure, that one goes both ways.
Some safes now come with an interior A/C connection for things like a GoldenRod and a remote hard drive for computer backup. Also handy for interior lighting kits, preferably LED so that you never have to change bulbs and won't break fluorescent tubes by bumping with a gun.
Flexible interior arrangements add convenience, but you'll probably customize it later to squeeze in more guns.
Storage on the inside of the door is useful.
Locking interior compartment is handy for ammo, jewelry, papers.
Size and number of door bolts is usually an indication of quality, but not always.
I like electronic locks for speed and convenience of access.