I've noticed that sometimes the domain name shown in parens next to the link is kind of useless. Take, for example, http://news.ycombinator.com/item?id=449670. "tumblr.com" is not useful in this case, but "titocosta.tumblr.com" would be more helpful--"oh, it's someone's personal blog named Tito Costa." Interestingly, it looks like sometimes you already do show more than just "domain.com", as in http://news.ycombinator.com/item?id=449221.
>>vegash+(OP)
That's been bugging me too, primarily with google.com.
To prevent wasted space you could ignore certain prefixes (www), or you could have a whitelist for hosts to show the prefix for (tumblr.com, google.com, etc).