TF2 is awesome, and what kind of marked it created "by accident" is quite remarkable. But hats aren't even a currency for every TF2 player. Some of us want them more for fun or for prestige than for trading. The actual value is different to everyone. There is no consensus since it's reliant on the personal taste of whatever interested buyers that are left.
So they are priced by rarity and popularity. An unusual is mostly priced after how popular their effect and the hat itself is, and how many and which game classes can wear it. You'll ask other unusual traders, who will give you prices that can go far apart. Actually selling for these prices is a whole different story. There's a lot of luck, timing, barter and patience involved. Six months later, that price can be completely outdated.
Even pricing guides are merely guidelines, what you really get can be far over or below what they say. So you can't pinpoint the value of anything.
A game update (like the recent "hat retirement" after which some hats won't be dropped anymore) can extremely affect the value of your hat, or devalue it without a warning. Getting an unusual is like a lottery, and every one that is dropped will affect the market to some degree.
Now I don't claim to know everything about TF2 trading, I'm not nearly enough into it for that. But still, would you trust this as a currency, when you don't have time and desire to get balls-deep into this market? A currency that has no pinpoint value, let alone security? It's like a lottery, and hats may be more like a stock than some dollars in cash. A stock that some people only want because it makes them look badass when worn on their heads.
Bitcoins I can see future for, since people have long been talking about digitalizing money.
