One of the biggest UI issues Bitcoin has, is that if it gets big enough, and even now, most people will have to pay for stuff like this: 0.000003 Bitcoins. I doubt regular people will really get used to this, ever. And even if you get to name those eventually miliBitcoins or nanoBitcoins, and you'll only ever deal with those, what was the point of naming them Bitcoin at the high-end?
If anyone makes another digital currency, they should make sure there are a lot more than only 21 million units. They should be counted at least in billions or even trillions.