Personally, I don't think there's any meaningful way that Danbooru can tag or otherwise indicate "safe for work". "Safe" is obviously a vague term, which is why we have the guidelines we do, to try and make it objective - but "safe for work" carries an implied meaning of "safe for your workplace", and that's a standard that varies immensely from one workplace to the next.
To go back to fossilnix's earlier example of "passionate homosexual kisses":
- in some places, it'll have no consequences (except maybe a flirty wink from the guy who saw it)
- in other places, it'll have no obvious consequences, but then you always seem to get passed over for raises or promotion
- in some places with "promoting immoral behaviour" laws, you may be fired if your boss decides to make an issue of it
- in other places with "promoting immoral behaviour" laws, you might get publicly flogged (and likely also fired)
Now admittedly, people in places like the last are not likely to risk browsing Danbooru at work, but the point is this isn't the kind of call Danbooru (in its vague collective identity) can or should be making; it's too dependent on factors completely out of our control.
Regarding the later direction the discussion took, I'm not opposed in principle to expanding the current ratings, but I think it could be tricky to do so usefully. I'm inclined to agree with Type-kun that it's more likely to just make things fuzzier: it'll be hard to avoid the impression that "safe" and "explicit" stay the same, but now there are two types of "questionable". (Also, as proven in a number of studies, humans get a lot worse at choosing when there are more than three choices.) Still, if anyone comes up with some solid proposed guidelines, it's worth at least considering.
On a side note (but still somewhat related): I didn't realise safebooru (the local one, not safebooru.org) was back, though it apparently has been for some time; I've been away longer than I thought, I guess. I seem to recall that in its original incarnation, it also avoided NSFW ads - I think by the method of not showing any at all. Ads have been gone on Danbooru for quite a while now, but issue #1703 (two years old, admittedly) suggests that they might come back eventually; is there any sort of promise that if they do, safebooru will still be limited to "safe" ads? I had a quick look through the forum, but couldn't find anything talking about it.