I think it’s perfectly precise and explicit enough: it means ‘this material may not be appropriate for you to view while you’re working’. That is very useful for a lot of people, who might view Diaspora while at work.
If you want to use specific tags for things such as porn, graphic violence and so on in addition to the nsfw tag which will hide the post from the streams of people who may not want to view it, that’s fine. But let’s not get rid of nsfw, which has a very specific and necessary role.