What Lom said is right. I can take an example out of Team Fortress 2.
There are 2 bats, same type, same skin, except one records kills. Yet the one that doesn't record kills is more rare, therefore, even though it has less use, it is worth a ton more than the other. Rarity sells, especially when a use is to make things look nice which I believe is the reason most of the blocks in Minecraft exist. (Clay/bricks have no use except to make things look nice, and they sell for much higher than most blocks)
What I really have noticed in this thread is the sheer lack of real concrete points. So, being someone who's been testing this trading for a few days, I'll provide what I know.
There is one trade on that list which stands out. I currently have a villager (not talking about this server, fyi) trapped in my house which has a deal, 20 Paper - 1 Emerald.
For every stack of sugarcane, I get 3.2 emeralds.
Now sugarcane is an item with a capping point, meaning it's not always in demand. There's a certain cap on how much you need, around the amount to make 15-30 bookcases and enough for speed potions, but here, it gets unlimited trade-in value. On a large sugarcane farm, I can easily get a full inventory of sugarcane every 3-5 minutes. I can craft this into paper and trade it for ~115 emeralds.
What can 115 emeralds get me?
Funny you asked, it can get me:
460 Bottles of Enchanting. That's three lvl 30 enchants, and most of the way to a fourth one. Netting anywhere from 3-10k credits from selling the enchanted items.
345 Glowstone. On a good day, I used to be able to get 1200 a stack for glowstone...for some reason. This would yield around 6500 credits.
800 Porkchops/Steak. Keeps me fed for a while, not all that expensive, but again decreases the value of food.
575 Glass.
4 Sets of Chainmail armor.
Over 1000 cookies.
The point I'm trying to make here, is that you can transfer a useless commodity (sugarcane/paper) into several other slightly more useful commodities, which cost more. This won't fix the economy, the only thing that will do that is to inflate the living hell out of credits. Which would be done by providing a high trade-in on diamonds/gold, but unfortunately that's been denied several times.
So you can clearly see here that you can get hundreds of emeralds every few minutes, and make well more money than mining diamonds. You can make more money farming sugarcane for 60 minutes that mining diamonds for a month.
~Sequel