It must be simple, unique, and easy to remember. That means that there are many factors what a great domain belongs to, including overall length, number of words, ease of spelling, and how much traffic goes there without prompting. Basically, .com domain name is pricy.
If you are going after a registered .com domain, you will have to contact the current owner, and even then, prices could be high for misspellings and multiple word domains. Before you allow yourself to completely fall in love with a domain name, create a few variants and backups.