I've just recently brought another one of my characters to gold-capped status, which leaves me the question: What to do now?
Well, I legitimately enjoy making gold in WoW, so of course I'm going to keep it up (although probably at a slower pace), but what's the point of having the gold if you're not going to spend it?
Gearing up your alts and mains is a good way to start. Most BoEs from the new raids, and crafted epics, are available for around 10-15k (with the belts being less, around 6k). Gems, enchants, and leg padding/spellthreads are another good source to improve your character. Pets, mounts, novelty items, whatever you can imagine can be purchased with your seemingly-infinite wealth.
I'm a mount collector myself, although I haven't been doing much collecting as of late. I'm at 93 mounts, currently, and unfortunately I don't have access to the Goblin Trikes as I'm Alliance side. So, in lieu of the non-existent Gilneas mounts, I'm purchasing a Vial of the Sands, for both myself and the crafter (as a sort of "tip").
I gear up my friends, give gold away for epic flying, and hold contests! I post in trade "First person to successfully guess my middle name, starting with a 'J,' wins 1,000g!" How do you spend your excess?