The single dollar bill. I've grown to dislike it.
Japan, the UK, Europe, Canada - all have coins for their one-dollar-like denominations. It's not that we don't have $1 coins in the U.S., it's that they're not popular and thus don't stay in circulation for long. I regularly listen to an NPR podcast called Planet Money, and they sometimes talk about this. Apparently people just don't like the coins; they'd rather have light bills rather than heavy coins.
The solution, of course, is to just ignore public opinion and discontinue the single dollar bill. This is what happened in Canada, and it resulted in successful $1 and $2 coins.
As for me, I don't mind the extra weight so long as the coins aren't huge. $1 coins would be a lot more convenient for vending machines, and it's a lot easier to just quickly stick a few $1 coins in your pocket when you get change than to pull our your wallet and put them away.
In Japan, there are six types of coins: 1, 5, 10, 50, 100, and 500 yen. I've written about how the 500 yen pieces are excellent for saving. Most people, even men, carry around personal bags in Japan, so weighing down your pockets rarely becomes and issue. Many wallets also have pouches for coins (which I don't believe is unique to Japan). And the 100 and 500 yen pieces are so easy to spend that they don't weigh you down long, anyway (the multitude of convenience stores and vending machines help see to that)!
Though I think the U.S. would do well to follow the lead of Japan and so many other countries, I'm not holding my breath. We're still holding on to Fahrenheit and resisting the metric system, after all.
What do you all say? Paper or metal?