You'd also need a system where you could blacklist people who abused it so nothing they set to urgent was treated as such.

Harsh but the boy who cried wolf and all.

Yeah it depends on who is emailing you. For Harj it is probably startups/ investors/ press etc, where there can be some social pressure upon using some kind of urgent tag only when it is genuinely urgent.

If most of your emails are customers who always want service right now then it is going to be a bit different.