Jeff wrote this on MSO:
The mouseover explains
question up arrow mouseover: "This question is useful and clear"
question down arrow mouseover: "This question is unclear or not useful"
Note that this contradicts the privilege page, which says nothing about clarity:
When should I vote up?
Whenever you encounter a question or answer that you feel is especially useful, vote it up!
I believe that "useful" is to be interpreted as "useful to someone", as opposed to "useful to you". Applying this to your situations, assuming that the question is clear:
- "This question is useful for me and I didn't know the fact that has been mentioned here." Upvote!
- "This question is not useful for me but I didn't know that." Upvote if it could be useful for someone else.
- "Why, in the world, would I, or anybody else, want to know that?" Not useful, so don't upvote.
- "I could have gone the rest of my life without knowing that." Upvote if it could be useful for someone else.
Basically, upvote if it's good for the site.
Of course, on meta, upvotes mean anything you want them to mean.