I see Flame's point but really come down agreeing with Piano and BBQ. There's stuff you can get by skill and stuff that you get by grinding. The "grinding" stuff that really just eats up time, I am happy to see a "purchase" option being offered in some cases. Person X might have a free weekend, person Y might have cash burning a hole in his/her pocket ... why shouldn't Bliz have an option for both? So long as the purchase isn't replacing "skill" items, I realy don't think anything gets unbalanced.
For collection meta-achivements ... it's not like cash options come anywhere close to closing the gap there. You still have to grind the vast majority of the activity. And there is always a component of such activities that can be obtained via the AH. Which means some people will get there via gold-buying, and some people will get via auctioneer wrangling (perhaps supplemented with paid-with-cash auction data). What's the bright line? I don't there is one that isn't subjective.
Overall, I think Bliz has been extremely conservative with their approach to micro-transactions and cash-for-item deals ... including Collection's Editions of the software boxes, Blizzcon giveaways, and all the TCG items, the total number of "RL buyable' items is probably not more than a few dozen or so out of the thousand of total in-game items. They could 10x that number, collect a ton of cash, and still not unbalance the game. But I think Blizz is ultra-sensitive about the perspective espoused by Flame that millions of WoW-ers share, and so they've held back. But you gotta imagine the fun conference calls with the Activision execs on this very topic ... the Bliz execs are willing to sacrifice their personal RL bonuses and political capital within their company to NOT just monetize the game rashly, and I totally give them credit for that...