You should look for extensions. i.e. .com, .net, .org or .biz Second look for the shorter name. Third the purpose of regestring the domain name. Thats all
Check out these domain buying tips . I think that should help you buy a nice domain http://www.domainrule.com/domain-buying-tips/
See if it has PR, see if it's been dropped, etc. Try Googling it to see if it's been involved in any illegal activity, scams, etc.
it wd be best if you buy the following type of domains and only dot coms -one letter -two letter -three letter domains
there are many things you should keep in mind whn purchasing an expiring domain. Like extentions. wether its 2 letter 3 letter or so on . its pr is valid or not. does the domain name mean something and is it pronouncable or not.
I think you have to think about what you want to use the domain for and target the domain for that purpose. If you are targeting products or markets its a good idea to use google keyword tool to find keyphrases to regaister as domains
It should be easy to remember, as short as possible, and no trademark issue Better to get a domain with keyword rich And get a domain that can be brandable..
I think .com for commercial purpose. .net for network, community. but .com is a great value if you have it first.
.com is always the king, try to find some general purpose keywords, even long tail ones, like 2 or 3 keyword. nowadays there are several companies looking for those names