My belief is that one exclamation point is sufficient for anything (except maybe announcing the end of the world or similar), and it is up to the reader to determine if what they are reading is exciting or indeed, extremely exciting. No amount of additional exclamation points is going to make an underwater basket-weaving class exciting to someone who hates both water and basket-weaving.
The other thing that gets overused a lot is all caps. Most people seem to understand that all caps can be interpreted as shouting, so they are sensitive to that. However, it is still used in promotional copy to convey excitement, such as, "ONLY 2 DAYS LEFT TO GET A 2011 NISSAN AT HALF PRICE." Almost every time I receive copy using all caps, it does not stay.
My biggest peeve? The use of all caps and multiple exclamation points together. Really? You'd really have to work hard to prove to me that something is that important and/or exciting.
Now if you are someone who is guilty of this practice, first, slap your own hand, then, understand this is not intended as a put-down, but more of a public service announcement to save the exclamation point. We all go over the top at times. In fact, I probably overuse the "Thanks!" closing in my emails because I really want people to know I appreciate them. I just draw the line at THANKS!!!