I see your point, I'm just saying I think at this time, people are used to those restrictions. I can't recall the last time I saw someone's username with anything exotic like punctuation
If you want to allow other stuff, you can, I just can't imagine many people taking advantage of it unless you put a big disclaimer saying "YOU CAN USE AWESOME SYMBOLS HERE"
Oh I forgot . I have seen several sites allow periods. I wish more would.
Another thing I'd add for the OP when you get to validating email address, don't exclude + or . in the part before @, those are allowed. I hate when a website tells me my email isn't valid when it is. But that takes me back to my rant about maiden names being longer than 3 characters.