An "Heirloom" means that the item usually has SOME sort of value, usually monetary, almost always sentimental and a HUGE lode of significance attached to it. It also means that if you get RID of it you will most CERTAINLY be in the dog house. Blizzard has gotten into the act with their "Heirloom gear" pieces that your HIGHER level characters can bestow upon your LOWER level ones. There are even rumors that in the not so distant future your characters from one faction will be able to share them with the opposing one as well.
Like all good heirlooms some are more desirable than others depending on what you wish to use them for and they may not all suit your taste and style. There are pieces for PvE and pieces designed for PvP. There are some which will allow you to gain more experience while you level and others which just make the experience easier over all. So how do you get them? What pieces are out there? Most importantly of all, how should you prioritize your purchases of them?
Heirloom items are purchased in basically one of two ways: via Emblems or Champion seals for PvE or using Stone Keeper Shards for PvP. The Emblem vendors are located in Dalaran, there are 3, one for each faction and then one neutral one. The Champion seal vendor is located at the Argent Tournament but you can only gain access to her if you are a Crusader. The PvP items can only be purchased when your faction controls the Keep in Wintergrasp. The differences in the two sets of gear are the stats on the items. The PvP items include the stat Resilience which is highly prized for Pvp but is not really helpful for PvE and PvE items ignore this stat for others.
Aged Pauldrons of the Five Thunders
Exceptional Stormshroud Shoulders
Exquisite Sunderseer Mantle
Lasting Feralheart Spaulders
Pristine Lightforge Spaulders
Prized Beastmaster's Mantle
Strengthened Stockade Pauldrons
Champion Herod's Shoulder
Champion's Deathdealer Breastplate
Mystical Pauldrons of Elements
Mystical Vest of Elements
Polished Breastplate of Valor
Polished Spaulders of Valor
Preened Ironfeather Breastplate
Preened Ironfeather Shoulders
Stained Shadowcraft Spaulders
Stained Shadowcraft Tunic
Tattered Dreadmist Mantle
Tattered Dreadmist Robe
Battleworn Thrash Blade
Staff of Jordan
Reforged Truesilver Champion
Sharpened Scarlet Kris
The Blessed Hammer of Grace
Upgraded Dwarven Hand Cannon
Bloodied Arcanite Reaper
Charmed Ancient Bone Bow
Devout Aurastone Hammer
Dignified Headmaster's Charge
Repurposed Lsva Dredger
Venerable Dal'Rend's Sacred Charge
Venerable Mass of McGowan
Inherited Insignia of the Alliance (Alliance only)
Inherited Insignia of the Horde (Horde only)
Discerning Eye of the Beast
Swift Hand of Justice
Tome of Cold Weather Flight (Requires lvl 68 one use only)
These are all the things we have access to at the moment but we know that even MORE are coming. Including but not limited to a Ring which will have a 5% experience buff on it and enchants for shoulders and chest pieces which will FURTHER increase the experience gain.
So which of these pieces should you get FIRST? That really all depends on you and what you wish to accomplish with your alts. Most will tell you if you are LEVELING an alt go with a set of shoulders first followed by a chest piece. These two pieces will give you the most bang for your buck in the quick leveling department with their STACKING experience buffs. Yep, that's right those buffs STACK so get those two pieces and you gain an additional 20% experience for EVERY quest and kill that you do. Personally I say look at your alts and determine what your long term goals for them are. If you have three plate wearers you want to level go ahead and get the Plate gear but if you have NO IDEA you may want to look at the cloth which will work for ANY character. Purchase pieces that will MOST benefit the majority of your alts and go from there OR the character you MOST what to focus on.
One thing to keep in mind is that just like with REAL Heirlooms the quality on these items very seldom can be beat. While you may not care to see your Death knight harbinger of doom running around in bright shiny Silver and Gold plate that they got from their goody two shoes older sibling just remember eventually they will be able to pass it along and clad themselves in as many blacken spikes as you could wish. Even quicker than you would have managed before.