I have been a bitcoin enthusiast, but I feel I need to play devil's advocate to balance some complacency and blind optimism I've expressed before.

I don't think bitcoin can succeed without significant improvement on user friendliness, per criteria proposed in this book:

http://www.amazon.com/Change-Function-Technologies-Others-Crash/dp/B000NA6U2O

For some, the current UI seems to be simple enough, but if you are serious about using it, you'll have to deal with the wallet file, a task I found to be nuisance even for computer geeks.

As a method of online/electronic payment, I always compare bitcoin's with Paypal, and as far as I can see, the pain of using Paypal does not exceed the pain of switching to bitcoin.

* Many online merchants accepts Paypal, very few accept bitcoin
* Being a non-reversible payment is a two-way street, it's an advantage for some but disadvantage for others.
* To recharge/withdraw money, linking paypal to a bank account is actually easier than jumping hoops with MtGox, Liberty Reserver and other equally hard-to-use intermediaries. (Coinpal/Coincard did a great job in this regard).
* To send/receive money from friends, without a mobile client, bitcoin is no easier than Paypal at current form.

The biggest advantage of bitcoin is its underlying de-centralized monetary concept, but without a real episode of hyper inflation, most ordinary people do not care this difference. 

In short, I really hope bitcoin will not become another great but failed academic experiment.