There can be some troubles with domains.
DOMAIN TASTING:
As already mentioned, domain registar can "steal" your domain, if you have search for it from their search field and you haven't registered it at the time. They register a domain for free, make a quick site, put Google ads on it and monitor how much revenue they can make. If they can expect profit, they register it officialy, if not, they made it available again without paying a dime. This is called domain tasting.
A little more
explanation.
DOMAIN TYPOS:
You've developed an successfull, very known site on a great.com domain and then you find out that there are graet.com greet.com , great.net also registered by someone else which takes away some visitors from your site. You should think on it on a beginning and register all main domain extensions and common typos.
Yaho.com, Yahooo.com , Jahoo.com, Yahoo.net ...
TRADE MARKS in A DOMAIN:
If you "accidently" use a trade mark of some big firm in your domain, you can expect your domain will be deleted. They will probably not bother
you with this but they will request a domain removal from your registrar. I mean - it's possible and they have a legal background to do this.
Mark alert is a paid option on a Domaintools.com
EDU and GOV DOMAINS:
So you want to register
.edu or .gov (
.GOV Registration - doesn't open) domain? Fine, but first check on their homepages if you fulfill conditions as a educational institution or a government organisation.
It was said many times that -edu/gov backlinks have a great value in SEO. May be - but this is because of their relevancy/quality and not for the extension itself.