Your granny can understand it, its simple, go to a site to buy them, enter address, buy them and get money, and to buy something send the money to the address with it and receive product.
The obnoxious thing about bitcoins, in my opinion, is acquiring them. Paypal was probably the simplest way until that option was taken away from us. Dwolla is easily the next best thing for the average consumer now, but transferring funds into Dwolla is a looong process. It takes about two days, in my experience. I haven't tested using Dwolla to transfer funds directly from my bank account to, say Mt. Gox, but I don't imagine it's much faster. (Correct me if it is.) Most people don't have Dwolla anyway, and once you have a Dwolla account why not just use
that to make a purchase? If you don't care about anonymity and if you're not looking to escape the fiat system, then for most purchases Dwolla makes more sense.
I think the killer application for bitcoins is fast and secure currency transferral. Right now it really isn't all that fast for most people as most people are trying to go from fiat to bitcoins to purchase. That process can take a few days at present. Two if you have a Dwolla account and need to transfer funds in. Four if you don't have a Dwolla account or don't have your bank account associated with your Dwolla account.
Best case scenario? I enter my credit/debit card info, the information gets verified, the card gets charged, I get bitcoins. I wager most people would be willing to pay a small premium to offset the cost of scammers if it meant getting bitcoins in their wallet within ten or fifteen minutes. It would be a boon for the bitcoin economy, too, as it would lower the barrier to entry for most people and would allow people to "strike while the iron is hot," so to speak. If someone catches the "bitcoin bug" and wants to buy bitcoins, they shouldn't have to wait more than an hour before they have bitcoins in their wallet.
Just my 0.02 BTC.
Also, the address system is a bit intimidating for most normals, I wager. I think web-based services which allow users to send currency by e-mail address or by name would be the best option for them. It would make the whole system a lot friendlier and a lot more pleasant to use.