The most crucial thing, for me, is to be able to pay for the basic, necessary requirements of modern life with bitcoins:
- Gasoline
- Groceries (including toiletries)
- Utilities & Insurance
- Clothing
- Household Items/Consumables (towels, batteries, candles, dishwashing liquid, etc.)
Toss in small "consumable" electronics like blank CDs, micro SD cards, etc. and I'd probably be able to reduce my reliance on FRNs by 90+%. Of course, you'd have to make the purchase process simpler than using Amazon through spendbitcoins.com to be competitive (probably not very hard.)
Now, some of this may not be too feasible. You can't mail gasoline, but if you're in the U.S., maybe there's a large gas station chain that sells gift cards? Also, shipping groceries probably won't allow for fresh bread or produce, and things like liquids (juice) may not be cost effective to ship, but surely some things could be sold. If you could also include long-term stored food, like they sell at wisefoodstorage.com, that would be great. Toiletries like deodorant, toilet paper, toothpaste, and household items like batteries and towels... those should be among the easiest to handle.
Utilities and insurance couldn't be done directly. But in my case, I could do just fine with you mailing me blank money orders with the amounts already filled in... both my insurance and utilities would take those.
Finally, least important of all to me would be the clothes, just because the hassle of not having a good look at the clothing quality and styles and sizes would have me make online purchases pretty infrequently. And it would probably be quite a hassle for you trying to stock all the options too. Although if you were to, say, just stick to an inexpensive, reasonably-well-known brand of comfortable jeans, I'd know in advance exactly what I need and would no longer have to shop for those locally either.