Technically:
- ease of use still a problem. Progress continues, slowly. Browser integrated wallets are key. One click purchase on any bitcoin accepting site is a killer feature.
- average joe, even average-geek-joe, simply cannot safely store a large amount of BTC. People are going to continue to lose large sums of bitcoin.

Culturally:
- I think a lot of people simply do not see a need for Bitcoin. I think to be honest, we need to ignore that sector. Too much effort is being put into marketing BTC to people who are happy with their credit cards. The largely ignored area is: kids. Technologically literate and malleable young people, who perhaps do not have bank cards, or for whom having a wallet on their smartphone will seem "cool" and futuristic. Simply installing a wallet on their smartphone without having to register or identify themselves to any bank? That monetary/spending independence should appeal to a teenager.

What I think will herald mainstream adoption is some service or app that appeals to young people and also serves a useful function. A killer app. Tipping and such on reddit is a small example; but that's primarily for the geek crowd. What's bitcoin's killer role in the snapchat demographic?

(but of course the killer "app" may simply be bitcoin as money, to buy anything anywhere, and so the hard part is still getting hold of bitcoin)

Practicality:
- buying it. Still tricky. I want to see vouchers you buy in a store (like mobile top-ups). I'd like to see buying BTC as easy for a kid as a mobile top-up. Go into a store, hand over cash, swipe their smartphone, and acquire their BTC. The very pessimistic view (which I think might be correct) is we'll have to wait for a parallel economy to appear so there is no cashing in or out. It may be all the AML regs kill any quick and easy bitcoin purchasing.
- tax complications. A company having to work out how they pay tax on Bitcoin income. Sure, if they use some service to convert automatically to $ or whatever it's easy, but in an ideal world they'd hold a balance in Bitcoin. I know I could run Bitcoin earning ads on a site of mine, but I avoid it because tax is annoyingly complicated enough as it is.