I wouldn't say the RMAH "legitimized" things...however, it did put one thing into the spotlight: the fact that idiots wanted to be considered elite so badly that they were willing to throw down hundreds of real dollars to buy in-game items without thinking about the consequences, the main consequence being that when you buy ENDGAME gear, you make FINDING that same gear less rewarding.
Let's say I buy an entire Zunimassa set on the AH, window shop till I find the exact rolls I want...then I find one Zunimassa piece out in the field, with rolls that aren't ideal. Obviously, it's gonna look sucky.
So far, I've bought one set piece on the AH, a Chantodo's Force, I think, couldn't have been all that great because I have other gear in my stash that I've FOUND that's better than that. But the point is, there are still a bunch of sets I'm yet to find, once I get to higher MP levels and higher Paragon levels.
People BUY gear (with either real money or gold), then they wonder why they aren't finding upgrades. Gee, I wonder why.
That's one big thing I never see when trolls hit the forums complaining that the game is unrewarding...how much time and effort was spent in the Auction House buying stuff they should've found. "But drop rates suck!" Yeah, if your MF, MP and Plvl are all stupidly low. Hell, even if they're low, you can still get good stuff. My level 53 Softcore Barb got to Hell this morning, found a Blackguard legendary. Not epic or godly, far from it, but it was still an upgrade. Most people would've found that and said, "crap, won't sell for more than 100k, if that." But it upgraded what I had on my weapon slot, and that's what mattered.
I like this game and look forward to expansions coming out. If the expansions do next to nothing to add any depth, then I guess at that point, I'll call it and start winding down my interest because the cap on how good it can get will have been reached. However, if the expansions give the game more depth, more quests, more things to do, more skills to try out, more classes to build and work on, etc., etc., D3 can be what D2 was. I'm confident.