You can't get great status updates in 13 characters!
There's also the problem of availability of unique status updates, the lower your character count the fewer unique status updates there will be. If you launch a service with only 1 character there will only be 25 possible status updates.
People want to be unique, but they're also lazy, so it's a compromise between the two. I think 14 characters strikes the balance.
25? Is that a typo or have I missed something?
Anyway, what's wrong with using numbers? Upper and lowercase? Unicode?
Damn phone, small keys big thumbs. I shall make amends for this terrible down voting accident post haste (can't seem to fix it).
But I would be afraid that somebody will figure out how to make a 13 character only twitter clone. This is a race that you can not win. Sooner or later some genius will make a service that allows you to broadcast nothing at all. Probably this service would even be free!1!.. killing all the other services in no time.