
I caved in today and decided to go back to my Mage, after levelling my Lock up to 74. I realised as I approached 80, just how much work I had put into the Mage at endgame in Wrath. Due to the introduction of achievements, my Mage has 74 mini-pets, and something like 64 mounts ATM. Some of those mounts and pets took quite an amount of time and in-game gold to obtain.
Additionally, I realised that my Warlock would likely not get into too many groups, simply due to things like having weak gear, having no achievements to allow entry into PuG runs, and being DPS meaning that I will be competing with others better geared and with better achievements.
Plus with the new 3.2 patch fast approaching, I'll be having a lot of trouble actually trying to play my Warlock through the endgame content, especially content I have already done on my Mage almost hundreds of times.
So in retrospect, even though Blizzard is trying to encourage people to roll new characters with the Heirloom items and experience boosts, I find that unless you are a chronic reroller, or are planning to make a character simply for gathering at 80, the reasons for trying to roll a new character are rather few.