I think about changing my username sometime so are there any character limitations of what can be used in a username? [ex. can - or brackets be used?]

• You cannot use any special characters except for _(underscore)
• You may only have one Underscore in your whole username.
• The underscore in your name must not preceed or succeed your name (IE username and username are invalid)
• No spaces or periods(.). Your name change will fail. Alphanumeric characters ONLY. (and one underscore between alphanumeric characters)
• Your new username must be longer than 2 characters. (Three is the minimum length)

That's from the ISS page.

I've seen someone with an exclamation mark in their name... has it been changed?

When a user signs up with a name they have a lot more freedom. That's where all the two letter UNs and exceptions to the above rules come from. Of course since Subeta doesn't allow multipule accounts if you didn't sign up with a 'special' Un, you can't get one.