Coins from the United States are so popular for a number of reasons:
Wonderful designs: In over 200 years of making coins, the United States has produced a plethora of lovely and interesting designs to entice the collector. The recent success of the 50 State Quarters program, the new Presidential $1 Coin program, and the millions of new collectors who have been brought into coin collecting are proof that interesting designs do matter to collectors.
Challenging series: Whether you're looking to collect mundane, common coins part-time or something more intellectually challenging, U.S. coins have it all, including many different denominations and types, inexpensive coins, super-rarities, big coins, small coins, and everything in between. The sky is the limit, but you can also keep your feet on solid ground.
Affordability: Contrary to popular belief, you do not have to be rich to collect U.S. coins. Just ask any kid who collects the 50 State Quarters, proof sets, and the Sacagawea dollar coins. Of course, having money makes your coin collecting possibilities much more diversified, but that's true of any serious collecting pursuit.
Availability of commemorative coins: You want commemorative coins? The United States has 'em. Since 1892, the United States has commemorated scores of national events, people, places, and organizations. If you choose, you can spend your entire collecting life just in this specialized area.
Availability of gold coins: If you like the yellow metal, the United States produced tons of coins made from gold. Many common U.S. gold coins are priced just above their metal value, making them very popular and affordable with collectors all around the world.
Active markets: Coins can be bought and sold at local shops, at local coin shows, at regional and national coin shows, over electronic exchanges, over the Internet, over the phone, through the mail, and so on. So something is always happening in U.S. numismatics.
Social benefits: You can meet lots of new people, and perhaps make a few new friends at coin club meetings, at coin shows, over the Internet, through newsgroups, and so on. Numismatists and coin collectors from the United States and all over the world love to talk coins and compare notes.
U.S. coins have some of the most beautiful designs of any coins of the world. In addition, the history of the United States can be seen on its coins. Through coins, you can see how the American colonies struggled to gain their independence and why a stifled economy due to a lack of coins was such a common complaint. Over the years, a young Miss Liberty has aged gracefully. A religious motto ("In God We Trust") was added to U.S. coins in response to the Civil War. A number of statesmen have appeared on U.S. coins, from presidents to other great national icons like Benjamin Franklin. Peace was proclaimed on silver dollars after World War I, and later coins honored conflicts in World War II and the Korean War. The United States has issued commemorative coins celebrating just about every event you can think of, from the founding of Delaware to the new 50 State Quarters to the Cleveland–Great Lakes Exposition.
In short, U.S. coins are steeped in a long tradition of history and remembrance. Collectors have happily contributed to the preservation of our common heritage.