Artirius blames it on the removal of attunements, but I'm not sure it's that simple. I'd say the primary thing is all these new dailies. With a minimum of effort, a person can earn somewhere between 200-300 gold a day doing the basic Isle of Quel'danas dailies. If you're a relatively casual player, someone who does their dailies, maybe a bit of tradeskilling, and a couple dungeon runs and a Karazhan run every week, you don't have much to spend your gold on. You buy your epic flying mount, and you're more or less done, especially if you're not the type to bother enchanting or gemming your gear to the max. Therefore, spending the money on dungeon runs is about the only way left for them to use it.
Similarly, since many people are abandoning gathering trade skills such as Herbalism and Mining in favor of using daily money to buy what they need and build a fortune, raiding guilds are left high and dry when restocking their guild bank for resist gear and consumables. In order to get the herbs, ore, and primals they need, they either must take time off from raiding to gather everything themselves, or they must buy it all at prices inflated by the glut of daily money. By a similar token, they may find themselves in tier 6 content long after they have most of the drops they need, looking for one or two final items that refuse to drop. By selling gear slots, they can make the cash they need to buy their consumables and build a repair fund without suspending raiding, and they only need to give away things their raiders don't need anyway.
So is this something that can be fixed? Is this something that should be fixed?
There have always been people buying raid slots -- or simply taking up spare slots in a friend's raid group. Back before Burning Crusade, Our AQ40 raid group had had no luck getting the Imperial Qiraji Regalia to drop, which was sort of bumming Alex out, since he had his eye on the Qiraji Augur's staff, even passing up other drops to save his DKP for it. Then he saw a druid in green armor in Stormwind, wielding that very staff. We weren't on Ventrilo at the time, but I swear I heard his cry of anguish anyway, and he certainly gave us an earful in guild chat, poor guy. By a similar token, I do recall seeing more than one instance of people in green gear with random pieces of tier 2 armor. So we can't completely say this is a new phenomenon.
There's a bit envy and annoyance in seeing people being able to skip months of wiping and progression and get a piece of gear that you've been struggling to get for months, especially if what's holding you back is the fact that it just refuses to drop.
At the same time, our druid friend with the staff proves that sometimes, in an MMORPG, it's as much about who you know (or what you can pay) as much as how much work you put into something or how much skill you have. That's probably a truth that isn't really going away, and even without a surplus of gold from dailies, we'll always have those people who leapfrog content. If you put in attunements, they'll buy their way through the attunements too. If you limit the gold they can get, they'll just take a bit longer to find it, or find some other way to get that spare raid slot.
At the same time, the fact that non-raiders are bored or rich enough to spend their money on raid slots like this is something Blizzard could probably stand to fix, and it would likely make the game more enjoyable all around. The dance studio and barber shop in WoTLK should help with this, but some more long term, expensive options like player housing or taxidermy would provide lots of fun money sinks that would probably suck up a lot of the extra gold in the economy pretty quickly, if implemented well.
Then again, they could just make level 80 daily quests give a lot less gold, too.