Coins have two levels of interfaces: a physical interface (their size, shape and weight) -- usable by vending machines and to certain extent by people with long-established habits of using these coins; and a visual interface -- used by people with less experience at coin-handling, such as tourists and just regular folks who don't use coins too often.
Now, the physical interface is "set in stone", and usually less important. Of course, it would be nice if coin's size was proportional to its value, but that's not always the case due to historical reasons. And changing current standards of sizes/weights of coins is extremely costly due to existing infrastructure of coin-operated machines.
The visual interface is completely another story. The single most important thing each coin must show is its denomination. Not pretty pictures. Nor ugly pictures. Nor head-shots of some random celebrities. Nor other nonsense.
Here's what I would consider a usable design:
Chinese coins aren't perfect, but still usable -- if I ever go to China, I could still count my change without knowing a single Chinese character:
Well, and here's what we are stuck with in US:
If you don't know English, you're screwed (even though a considerable part of population is actually Spanish-speaking). If you do know English, but never used these coins before, then it'll just take forever to count anything. If you are an American, and you used these coins your entire life and think they are perfectly fine — then see which of these two arithmetic problems you can solve faster:
What I don't understand is WHY. Why there are no attempts to improve coins design. It's not like the QWERTY keyboard or miles/Fahrenheits issue, where users habits outweigh any possible usability gains. I don't believe that changing the eagle to "25" on a quarter would suddenly create nation-wide confusion...