I amassed most of my fortune through GDKP runs at the end of WotLK. By the time Cata was rolling around, I was running 3 25-man ICC GDKPs every week on my alts, and buying all of the BoEs to flip for profit. I've been slowly losing money in Cata as I gear up my many alts, but I'm still sitting on quite a large sum of gold with no real threat of running low any time soon.
Getting "rich" in WoW isn't at all difficult. It requires some addon know-how, and a lot of patience, but anyone can do it if they feel like it. People who have no gold seem to think there's some secret formula that we rich folk keep under lock and key, but the truth is we just put in the effort and are now reaping the rewards. I put in quite a lot of time at the beginning of Cata leveling alts and professions and setting up my addons to get my business up and running. These days I only spend about 45 minutes per day playing, and I make 3-5k gold every day doing my JC dailies, selling glyphs and LW bags, and flipping rare gems. If I had the patience to log in and relist my glyphs right before raid time, I could make an additional 1.5-3k gold per day... but I'd much rather be lazy.