1. A Gladiator Title + Mount
One of the pinnacles of arena PvP is the gladiator title and mount which is given to the top .5% of arena teams in each battle group. Getting one of these rewards is a huge badge of prestige and means that you are the best PvP'er on your realm. Sadly because one I am not a huge PvP'er and two the fact I just do not play at that echelon I will never achieve one of these rewards.
2. A Server First
This is in the same vein as the gladiator rewards. To have a title like Death's Demise or Conqueror of Naxxramas is a huge achievement. It means that you beat the best guilds on your server to the hardest bosses available in the game. You can wear the title proudly around your sever and say "yes out of all of you I am the best". And much like the gladiator rewards I do not have the time t commit to a hardcore guild like the ones that race for these. Now don't even get me started on world firsts....
3. The Ashes of A'lar
The final thing I would like in game is the Ashes of Al'ar mount. This is a gorgeous mount with a graphic only recently shared by the Dark Phoenix of Cataclysm. This mount drops from Kael'Thas in The Eye instance of Tempest Keep. It has a minuscule drop rate of a mere 2% much like Rivendare's Deathcharger (which is actually slightly lower at 1%). With a drop rate like that I'm gonna have to be extremely lucky to ever see its pretty purple text.
Thanks to Blog Azeroth for the topic!