Adoption. The more people that own bitcoins, the less volatile they will be.
What if 100k of 1m people are as fickle as 1k of 100k people when doom&gloom or sunshine&roses reports are made, then repeated by other news outlets?
If Walmart accepted Bitcoin today, it would be the "start of Bitcoin."
If Walmart stopped accepting Bitcoin in a year, it would be the "end of Bitcoin."
Should a large handful of major B&M stores accept BTC, it still only takes a few months of "Bitcoin is crashing, doomed to failure" for them to abandon Bitcoin. Once one drops out, they'll all stop dropping out, and that's the "end of Bitcoin." It's like HD-DVD vs. Blu-Ray. Once a couple major organizations dropped support, Blu-Ray won. Currencies we're used to don't have this problem, because the government forces everyone to accept that one currency. If I took out a BTC loan from - Idunno, AmericanJohn... he would necessarily need to accept USD for repayment in a court of law. He can't say "Oh, no - you can't give me USD. I gave you BTC, and that must be returned." The judge will tell you to GTFO before he rips up the paper giving you any reparation at all.
Bitcoin can always be replaced by something new and can always be abandoned. It creates a lot of uncertainty.