Well it sounds like you're a little more on the fence than your original impression of the gold ah taking a huge hit ;p
I guess I've always been under the impression that the gold AH *should* be preferred by the player base at large, while the RMAH was used for a specific few who wanted to buy and sell with real money.
I don't see any really good reason for the player base to prefer the GAH at large. I feel like it's only being propped up by people maintaining the mental separation between in-game items and the rest of the finances.
If you look at the incentives, at the launch of the game there is going to be huge demand on the RMAH and very little supply. The belief that prices on the RMAH will never be higher than they will be when the game launches is not unfounded. I worry that alone is enough to make the RMAH the go-to place to sell, and therefore buy.
Granted the GAH can't become totally irrelevant; if you want to convert non-gold items to gold it's obviously the place to do it, and even with a dominant RMAH the GAH will still track the RMAH closely - otherwise there would be arbitrage opportunities.
I just don't see it as very healthy for gold to have a low time value and for Blizzard to lose control of their economy.
This mechanic is actually what would have happend under the old $1.25 fee. All the better items would go to the RMAH because there's much less tax--- no doubt about it. This would force a lot of people that normally wouldnt deal with the RMAH to make the gold/item to money conversion to trade for anything worth more than $5. I see this 15% as a *solution* to this problem.
It's possible it will work out that way - that top quality items will naturally go on the GAH and people will have to 'cash in' to bid on them. It's also possible for even small items to end up on the RMAH now and the GAH becomes an afterthought.
The issue for me was that the percentage of items that made sense to sell on the RMAH was going to be small, and was going to diminish with time, while we had a more or less built-in guarantee that the GAH was going to be the main auction house for most items. This also naturally partitioned things the way they were likely to go anyway. So it guaranteed that both auction houses would be relevant and interesting in their own right.
The new numbers more or less guarantee that one of the two will not be interesting, though it is unclear which one. I don't see that as a good thing at all - not only would I not want to leave anything like this to chance if I was in their shoes, but there's little advantage to doing it this way over just picking one auction house or the other, and shutting the other one down. Leaving them both open, in this form, is enabling a lot of false choice and wishing for the best, which is not good design at all.