Quoting what "paly155mm" said in the comments on the item http://thottbot.com/i42944 and on other heirloom pages:

"Gold items aka Heirloom's
-Bought with Emblems of heroism\Stone keeper's shards.
-Can be mailed to characters on the same server and account.
-Can be enchanted with original wow enchants ex. crusader.
-Have no durability and their stats go up as you level.
-The shoulders experience bonus is applied to experience from killing mobs, but not quests. (Changed: The bonus now includes quests.)
-Shoulders adjust for level restricted armor type ex. plate Polished Spaulders of Valor will change into mail for a pre level 40 paladin or warrior."

To understand what heirlooms are, I've quoted Wikipedia, with a little twist:
In popular usage, an heirloom is an item, perhaps an antique, that has been passed down for generations through characters from the same account and faction.

I've tagged the "binds to account" items that I could find.

The experience bonus from the shoulders and chests stacks, and so does the trinkets with their health and/ or mana gains.

Just thought I'd add in here, they CAN be enchanted with 'chants other then Crusader, but they can't have a level/ilvl limit.

So, what u see when you are looking at the stats through your 80 toon is not what they will be when you put that heirloom on the lowbie toon? Well, that kinda takes a bit of fervor out of trying to get them. Oh well, I dont have alot else to do so I guess I will do it anyway, haha.