I think the greatest contributing factor will be the ability to pay for a McDonald and a beer in BTC. Why? Because all means to exchange BTC with cash (exchanges, Bitinstant card...) will always be subject to more and more government control, and governments have all the cards and all the guns. A real, strong, parallel black economy in BTC will be unstoppable and uncontrollable.
I dont agree. You see buying a beer with cash is one of the places where cash is at its best. The government really isnt much interested in what kind of beer you buy or how many McDonals. Nobody is in fact.
This will be the last place bitcoins will ever be used. There will of course be meetups for enthusiasts where this happens, but normal places taking Bitcoin as a normal thing will be unusual. What I am saying here is that there will be places that do, but it wont be run of the mill for every place untill Bitcoin has truly conquered the world.
Bitcoin has certain properties that make it very very attractive for certain uses - internet, transfer of money over distance, even investment, but buying beer is so easy anyway, that I just dont see the advantage here.