The hashtag isn’t a technology or even a platform service like the Facebook Poke. It’s more of an organizing principle, a way of opting into a larger public discussion. To get metaphorical: It’s more about the signal than the wire. It’s a good idea and it works. Everyone should be able to use it. And to Twitter’s credit, it’s not trying to lock down the hashtag. It hasn’t filed any patents or groused about other services ripping them off.
Which orphan keyboard character shall we rescue next? I vote for the tilde (~).