Good Idea but I really doubt how many people would actually select that when solved and if its mod that they have to do it then it would be of not much value
Well, I agree since its something new, it'll take time for people to get use to this, but then this will add a great value to this site, since just by looking at a thread we can say it has been solved. It would improve queries being solved to a great extent.
A new idea always take time to be implemented, but once people get use to it, they do it automatically. Like the thanks button which is still catching up, but soon everyone will get use to it

As for the mods doing it, yes it may happen at the start, Specially with the newbies who join just to ask one question, but we'll have to help people and let them know there is a feature like that.

I think its worth trying...