It touches on exactly what you said none of them do. RMAH places real value on items. It's an official value... not contrived based on a black market most people never took part in. It's now a supported and encouraged part of the game.
Ah, no. First of all, it explains how it's beneficial to Blizzard, not how that had any impact whatsoever on any design decisions Blizzard made. Secondly, that value only comes into play if people are actually willing to pay it- there is no Real Money Trash Vendor.
Furthermore, it's not the value that's set, it's the exchange rate. It dictates that gold is worth in relation to real money, not how much of either anything is worth.
Ok, let's do some quick math and logic here.
If the price of gold is set based on the exchange rate of 100,000 gold per $1 (arbitrary value) and you find an item that sells consistently for 1,000,000 gold in said auction house, does that not mean that that item is worth $10? Yes, of course it does. Thus, RMAH places actual real worth in gold and items. Whether or not YOU IN PARTICULAR are affected by that is not even the argument. If Gucci wants to place a value of $5000 on a handbag, that's the value, and people will buy it.
Now... how it affect design decisions? The decision to make something hard to get creates artificial low supply of items in that tier. So the harder the last level is, the harder it is to get items, driving the RMAH price up increasing Blizzard profits. If they want some item to sell more, they need to lower the prices so it becomes more attainable. But not too much, otherwise it will be too easy to get. If the last level was easy, everyone would have a good chance of getting items in that tier and they wouldn't be as active in the AH. Also, pushing people to the AH is the "drop rate" on particular stats. If the game benefits from life stealing items and they notice the price on those items is drastically low they just decrease the drop rates and the price starts to go up. Now, I've heard/read people talking and most people I know of find it much easier (and we all know people will take the path of least resistance...) to farm gold by selling off junk (Real Money Junk Vendor) and just buy the item they want from the AH. It's far easier to let someone else farm and list the item and just give them gold.... I mean money (because gold == money now.)
But who cares if the harder level is too hard... don't play it, right? You ever wonder why the lower levels are so easy? There's no challenge there and people will not feel their money's worth. Let's create a HUGE cliff of difficulty so people will be desperate to break that barrier... some of them will pay money because it's not worth their time to grind out gold.
Or are you talking non-item design choices? Well, that's easy too. Some skill you have lets you play the game without needing some bad ass loot only found on the hardest levels of Nightmare... nerf it. People will be enticed to the AH to buy better gear. That's a hell of a design decision change... they'd never nerf something so much... Force Armor + Diamond Skin? Hello? Oh, you say they can just go to a lower level area and farm some gold to get items to fill these needs?
Did you even read any of the links I posted? They nerfed gold drop rates. So now that gold is hard to get,
how will you afford the new higher repair rates and still have fun with the game? One person even suggested selling off items on another character and getting rid of it! Wow. That's what I want to do just to be able to play a stupid game.
It's beneficial to them to make things rare so they fetch a higher gold value and entice players to pay that price. It's also beneficial for them to control the cost of items so that it doesn't get too obscene so players will still pony up. It's also beneficial for them to create artificial scarcity to create those Bentley/MErcedes items that power players will crave. If they find the item is not selling, they increase the drop rates to lower the price and keep people buying and selling on the market. Do you really not understand how economies work and how Blizzard could absolutely be using this to their advantage to make money on the RMAH?
RE: Peer Pressure. You apparently have never been outside. Peer Pressure can be huge. You see someone driving around in a new car and it looks totally awesome or has some cool features you like. You have some urge to own that item (even if you try to claim you don't.) It's human nature, deal with it.
RE: Blizzard made auctions. Prove they don't do it. A statement is one thing, but Reddit founder recently come out and said they made fake accounts to entice people to use it.
The Bandwagon Effect... you should probably read about that too. Also, just because YOU don't fall for some trick doesn't mean that applies to everyone. Blizzard making a gameplay change or secret auctions can most definitely impact how the RMAH is perceived. I just read the other day that people were making a big deal out of actually making money on the AH... that has a psychological impact on people. 1. Money can be made. 2. I can buy things with real money and not be derided like before. Who's to say Blizzard doesn't buy out items to create the image of demand? A few hundred bucks here and there will not hurt their bank, but it will definitely make people think people are paying real money for digital items. You can call it a conspiracy if you like, but it could very well be happening and you'll never know about it.