Each city quartermaster has a few very unique items to offer. Among these are tabards, non-combat pets, and special mounts. Let's take a look at some of these, because the unique offerings may be what influences your decision on which city to champion first!
Exodar
The unique non-combat pet you can get by being a champion of Exodar is called the Amen Vale Lashling 
(click link for image). It is the same basic model as all other lashling/lashers in the game, only cute and miniature! Costs 40 Champion's Seals.
As for the unique mount, it is an Epic ground (150 riding skill) mount called a Great Azuremyst Elekk . Costs 100 Champion's Seals.
Darnassus
For being a champion of Darnassus you can choose to purchase the Teldrassil Sproutling. The simple name belies how cool this pet is - It looks like a baby tree druid, complete with the falling asleep animation and doing the tree dance! Costs 40 Champion's Seals.
The mount offered by Darnassus is a Swift Darnassian Mistsaber , which is a light gray color with a blue harness. Costs 100 Champion's Seals.
Stormwind
If you choose to become a champion of Stormwind, you will have the option to purchase the Elwynn Lamb non-combat pet for 40 Champion's Seals. This little guy is so cute that we can overlook the fact that clicking on him 50 times won't make him explode.
The Swift Elwynn Steed can be yours for 100 Champion's Seals if a heavily armored horse with green glowing eyes is more your style.
Ironforge
A champion of Ironforge can be parent to a little black bear cub if they want to spend the 40 Champion's Seals on the Dun Morogh Cub .
The Swift Ironforge Ram is the epic ground mount offered for being a champion of Ironforge, available for purchase for 100 Champion's Seals.
Gnomeregan
The Mechanopeep is an extraordinarily unique pet available to Champions of Gnomeregan. This cute little mechanical pet looks a lot like the Gnomeregan mounts while striding along behind you.
If you prefer the larger version, you can spring for the new ground mount, the Turbostrider.