When I try to perform a search in the Auction house, it does not allow me to submit my query without a specific desired quantity. Why is that? (I was searching for crafting items)
The process of purchasing commodities - crafting items, gems and (in the RMAH) gold - is a complicated one. It's not that there's just a listed price per unit for each commodity. Instead, the system automatically attempts to obtain the quantity you desire from the cheapest offering available, and if the items under that price run out it moves to the next cheapest offer, etc.
In practice, it means that the price is very dependent on the quantity you are interested in, and without knowing that the system cannot provide you with an accurate quote.
As ThiefMaster said in a comment, this is likely an attempt to combat speculation.
Imagine the following situation: you have a high-level gem (say Star Emerald). You check the AH to get a feeling what's the fetching price, and discover that there are currently 5 of these, averaging at about 100K gold per piece. You buy them all (asuming you have the purchasing power) and relist them at 150K a pop. Since the supply is essentially zero at this price (you bought them all), and it's unlikely these are commonly listed, you are effectively inflating the prize of the gem. At 150K it's still probably cheaper to buy these from the AH than to roll your own from the jeweller, so you can expect a nifty ~30% RoI (ABT) when these sell eventually. You can continue to watch the supply and buy out gems that are listed at lower price than the one you set, and relist them again at your selected price level.
Lather, rinse, repeat.
With enough purchasing power, you can corner your own piece of the market and set the prices higher than what the current supply and demand would suggest.
Were you an actual stock trader, SEC would be very interested to hear from you for violation of Securities Exchange Act.
ABT - After Blizzard Tax. Revenues adjusted for the Blizzard transaction fee, currently sitting at 15% of the selling price.
More likely, it's because Blizzard intends the Auction house to be terrible until they release the currency auction house.
This way when the currency auction house is released people will react more positively to it.
Forcing us to search by quantity is easily one of the stupidest interface decisions ever made in any implementation of anything like an auction house in any game in any genre in the history of video gaming.