I'd say the symbol needs to be something that appears in common keyboard layouts. Personally I'm fond of "\" as it's used in a lot of other cases. % also works, but as it's the standard escape character I have a few doubts about it. I really don't like using # as a line end (though I guess technically, that's how it works now).
I'm not entirely sure the first part even needs to be fixed (honestly, I'm surprised we allow mid-line comments, but it seems to me the current method of doing it, even if it will catch people by surprise the first time it happens, is actually slightly more flexible overall once you know it exists).