You cannot possibly be a verb!
"Thank you, Bill" is correct. If you wanted to be uber-pedantic, the full sentence would be "I thank you, Bill.", where I is the subject, thank is the verb and you is the object. The omission of the subject is accepted usage, just as in "(I wish you a) Good morning".
As for the exclamation mark, that conveys the sense of emphasising the sentence, implying that it is not heard every day. "Thank you, Bill!" could be construed as "My goodness, Bill - I never thought you'd do that. Thank you so much, mate! What a surprise!" Etc.