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.
2. They are easy to remember
Many Internet users do not use bookmarks. They just memorize the domains of their favorite websites and type them whenever they wan to visit one. Guess what, if your domain is complex and not easy to remember you will lose these visitors along the way.
Example: Brcwr.com is a short domain name, but is not easy to remember at all, so it would be a bad idea to use it for your website (unless the initials represent the name of the website or a memorable message).
3. They are easy to spell
The last thing you want is visitors misspelling your domain and ending up somewhere else.
Avoid unusual foreign words, words that have complex pronunciation, strange combinations of letters and anything else that might cause someone to misspell your address.
Example: CappuccinoBar.com might be problematic for English speaking visitors. Cappuccino is an Italian word, and not everyone is aware where the doubles are placed.
4. They have a .com extension
Organizations might prefer to register a .org domain, and companies targeting very specific geographical regions might want to register a local domain (e.g. .it, .co.uk, .cn and so on). Apart from these cases, however, a .com domain is always the best way to go. This extension is the most popular around the around, and it is already stuck in people’s mind.
Visitors coming to your site via search engines or organic links will pay attention mostly to the name and not to the URL. The next time they want to visit your site it is very likely that they will just type its name followed by a .com. Guess what, if you are not there when they hit enter they will just go somewhere else.
Example: Darren Rowse created his popular blog on Problogger.net. Despite having a strong brand, some visitors were still going to Problogger.com. After a couple of years Darren decided to buy the .com version for $5,000 and redirect it to his site, so that no more visitors would leak.
5. They are descriptive
Many visitors will come to your site through the search engines and via direct links on other websites. That is, they will come if the domain that they will see will be appealing.
Having a descriptive domain name will give visitors an idea of what your site is about even before they enter it. If related keywords are present in the domain it might also help your search engine rankings.
Example: You would be able to guess what TelevisionGuides.com is about even before visiting it right?
Put it in another way. Suppose you are searching for a movie review. You make a quick search in Google. The first result comes from MikesLair.com. The second result comes from MoviesCentral.com. Which one would you rather click?
6. Or brandable
A brandable domain will have a nice pronunciation, an interesting combination of letters or simply an appealing visual effect. Sometimes they will not be descriptive, but they can be equally efficient.
Brandable domains will make your visitors associate the name with your website and its content. (Notice that brandable domains can be descriptive at the same time, but that is not always the case.)
Example: Kotaku.com is one of the most popular gaming blogs on the Internet. The domain is not descriptive at all, but the brand is so strong that gamers immediately recognize it across the web.
7. They don’t contain hyphens or numbers
Domain names containing hyphens and numbers are cheaper for a reason. They suffer the same problem of domains not using a .com extension or with complex spelling.
Consider Tech-World.com. The names that will stick in people’s mind are “tech” and “world.” Many visitors will just forget the hyphen along the way. Eventually they will try to access your site by typing TechWorld.com, in vain.
Numbers, on the other hand, will confuse people with the spelling. Suppose you registered Tech5.com. Visitors might mix it with TechFive.com, if they manage to remember the number in the first place!
Example: Coolest-Gadgets.com is an extremely popular gadget blog, with over 70,000 RSS subscribers. With such a huge readership you get people often typing the domain directly on the address bar. Needless to say that many of them would just forget to add the hyphen. The owner of the site bought CoolestGadgets.com afterwards to fix the problem.
Do not get discouraged if your current domain doesn’t have all these characteristics; or if you can’t find one that does. These are just factors that you should consider when evaluating domain names.
There are plenty of examples of popular websites with domain names that lack in one or two points covered on the list. Just make sure that your domain has most of the characteristics and you should be fine.