What makes a good domain?
Is the domain clear? Does it make sense?
Is the domain short (1 or 2 words; 2-3 characters)?
Is the domain a product or service?
How many people are searching for keyword(s) in your domain?
Can this domain be developed into a business or be used as an asset to further an existing business?
e.g. DJ.com Shoes.com, WebHost.com, WeddingPlanners.com, etc. are all good domains.
What makes a bad domain?
The domain is too long.
The domain is not clear.
The domain contains too many hyphens.
etc.
Edit: This of course isn't a rule as I'm sure their are exceptions, but your gut should tell you what is a good domain and what isn't.
Ask yourself: would I spend thousands of dollars to buy this domain to help my business or to build a business? Be brutally honest. If not it's probably not as great a domain as one might think.
Just my 2 cents.
Skinny