Definitely should relate to the business [SEO and all that] and being easily remembered is high on the list too.

Two others I'd throw in...
Shouldn't be too long, otherwise you fall foul of typos.
Should be easy to quote over the phone. In other words, avoid hyphens/underscores if possible as they are tricky to describe to a computer-illiterate. Also numbers can cause confusion [I wonder how many have tried to get into 'otwo.ie' instead of 'o2.ie'??!!]