1. They are short
Good domain names are short. It is not a coincidence that all the three-letter and four-letter .com domains are already gone, and that the five-letter ones are going fast as well.
There is no definite number of characters that you should aim for, just remember that the shorter the better. If you really need some guidance, try to go below 10 characters, and never exceed 20.
As for the number of words, one-word domains are gold, two-word ones are good, three-word domains are average, and above that it is usually a bad idea.
Example: Quotes.com is a superb domain and probably worth millions of dollars. ProQuotes.com is a good two-word domain worth thousands of dollars. ProQuotesNow.com is an average domain and could be used for a website. YourProQuotesNow.com is plain worthless.