Principles of Minimalist Wallet Design
Materials
Thinner is better, as long as it doesn't compromise durability. Wallets have layers, and layers add up fast.
Durability matters a lot. A good wallet should last long enough for me to tell dozens of people how much I love my wallet, without being a weirdo who likes to tell people how great his wallet is.
Leather and similar materials have great hand feel, but they're thicker. Use the thinnest material that has the other properties you need. One of my favorites is the rip-stop nylon used to make kites and spinnakers.
Some wallet makers use slabs of metal or plastic. They tend to be around 3mm per layer thick for durability, making them unsuitable for a slim wallet.
Layers
The more layers of material in your wallet, the thicker it will be. Instead of having separate pockets for each card, let cards lay together. They'll be harder to separate, but most cards are rarely used.
Bi-fold is usually going to be better than tri-fold, because twice as many layers is about 50% less than thrice as many layers.
Optimize for the size of the full wallet, not for the size of the empty wallet.
ID windows don't help. Usually if someone needs to see your ID they want it out of the wallet anyway, to inspect both sides or to verify anti-counterfeit features. The plastic may seem fairly thin, but it all adds up. Often the ID pocket on a wallet is the hardest one to remove a card from, so again it's just not helpful.
Seams
Seams also add to the thickness of an empty wallet, but only seams that cross a card add to the thickness of a full wallet. Seams at the edges don't matter.
Angles
Cards should lay flat. If the seams in a wallet cause a card to stick out at even a slight angle, you've got a full wallet that's thicker than it has to be.
Security
I'm not talking about tracking devices or blockchains, just make sure my cards won't fall out of my wallet when it's in my pocket. Usually that means a folding wallet, with the cards open into the fold. Tension can work instead, but it limits the practical capacity and durability of the wallet.
Cards falling out when I open the wallet can be a problem too if opening doesn't naturally put the cards into a position where they're unlikely to fall.
Capacity
The first step in minimizing your wallet has to be to stop carrying cards you don't need. A driver's license, a credit card, an ATM card, and maybe a bus pass should do it. If you must carry less frequently used cards, like gift cards, loyalty cards, insurance cards, and library cards, put those in a separate wallet that stays in your purse or backpack.
A minimalist wallet's capacity shouldn't be much more than 4 or 5 cards. If there's ample space for 6 or 7 cards, it's just a normal wallet.
Cash is often unnecessary, but it's good to have a way to store a few bills and, temporarily, a receipt or two. Cash takes up more space when folded, so if you include space for it, make it flat or folded only once.
Features
None. Minimalist wallets do one thing and do it well. They hold cards, and sometimes cash, and that's it. Metallic layers for RFID blocking can be extremely thin, and a stranger really can skim your card from your pocket without touching you, so I'll give that one feature a maybe.
If you include RFID blocking, test it. If a phone can read a card through the material, it's not blocking anything.
Price
My favorite wallet costs $60 and I guess I'm willing to pay that, but a minimalist wallet ought to use minimal materials and be fairly simple in design, so that feels like a bit much.