agreed. although slots on pants and chest armor are not extremely important IMO, you can also get the stats you want on those without the sockets...It's kind of hard to say, because many of the stats/affixes depend on what else is on the item, and what class is looking to buy the item. For example, +Armor can be great, but without other good stuff it's probably not going to sell. Or, people love buying high primary stat with good VIT, but high VIT by itself typically won't sell, and high primary stat by itself does at least have a (small) chance to sell. Again, it's hard to explain all of the different exceptions by item type, what's on the item, and what class it's for, but there are some stats that do stand out.
Increased Attack Speed
Critical Hit Chance
Critical Hit Damage
Max Slots (3 for Chest, 2 for Legs, 1 for Head, Amulet, Ring, Weapon, Off-Hand/Shield)
Primary Stats (DEX seems to still sell the best/for the highest prices)
And then these are pretty decent if coupled with the above:
Life on Hit (mainly on weapons around 600 or higher, but if really high on amulets/rings that's good too)
Magic Find (this is probably going to be more sought after very soon so I figured I would include it)
Movement Speed (mainly just on boots, and only if it's in addition to the above stats)
Basically, the more of these you find together on an item, the better. As for "what items are the most rare," I'm thinking you mean which ones do people want the most to find with good stuff on them? That would be (as far as I know) Amulets and Rings above all else, then Gloves, then Helms, and maybe even Bracers at the end (sure why not, lol.) I believe the reasons for this are:
Amulet and Rings are the hardest items to find with great stats on them (making them the most expensive items in the AH)
Amulets, Rings, and Gloves can roll IAS, Crit Chance, AND Crit Damage on them ("trifecta")
Helms and Bracers can roll Crit Chance, but Helms can also roll a socket which can be very useful
So yeah, hope that helps, and if anyone sees this and thinks I've got something wrong, feel free to correct me.