The driving force behind Bitcoin is illegal activity. Silk Road drugs, China exchange control evasion, and, in the background, gambling. Without some new large-scale illegal or semi-illegal activity adopting Bitcoin, the overall trend is down.
Not really. The driving force is holders. Holding a single coin consumes as much supply as a huge amount of economic activity if you assume a reasonable but still very conservative velocity. For example if a transaction ties coins up for 3 days on average then holding has 100x the effect as activity.
What's the driving force behind holders?
First, liquidity. Its important to be able to easily spend BTC. This is why exchanges are important and why other economic activity (legal or illegal) is important. Liquidity, portability, and easy-to-hold-yourself are the key arguments to prefer BTC over gold for holding (its a PITA to redeem your gold coins for USD or food). So removing liquidity removes a big advantage, although not the only advantage. So Gox was a big blow and conversely the coinbase exchange is a shot of liquidity confidence for US markets... but the real confidence comes from seeing consistent economic activity occurring. So if you want BTC to succeed, spend your coins (and rebuy them)! While its obviously less convenient (than other forms of payment) if you buy exactly what you need just before you spend, I find it faster and more convenient if I load my phone once a month (say) and then spend when needed. I can point my phone at a QR and press pay a lot faster than getting out my CC and typing in all the numbers...
Second, diversification. Holders want to move some assets away from their home country currency. And by extension move some assets away from all currencies using the same potentially flawed management system (central banks). But stabilization of the fiat systems means that pressure to diversify out of fiat is pretty low right now (I am guessing I haven't done research). And for those doing it, Bitcoin is not comparing as favorably against stocks, real estate, gold, etc because of the year long BTC price decline. However, that vicious circle can reverse quickly with 3-5 months of price stabilization or slight rise (as we saw in 2012). And recent behavior in Greece might bring the flaws of the global fiat currency systems to the forefront once again, depending on the repercussions of the 240 bln default has. Remember, QE etc (essentially buying loans with newly printed $) has no long term effect on inflation (in fact, causes deflation), because eventually the money gets paid back. Once paid back, the QE printed money is retired so net is 0 (actually net is negative given payment of interest on the loan). Works great UNTIL the loan defaults. At that point, the QE money will never be paid back -- so there's now 240bln additional euros permanently in the supply. Oops.