There's something about forced politeness that makes everything sound less enthusiastic than you actually are. Let's face it, when you are scripted to say, "I would be great for this position, I look forward to hearing from you," it's hard to make sure that they truly understand that I would be awesome for this position and I'm crossing my fingers that I hear back soon. Because you can't write that. You have to be eager but not desperate. Available but not at such a loose end you appear undesirable. Seriously, the fine points of the art of flirting are more easily understandable than the rules and regulations of cover letter enthusiasm.
I find that the hardest thing is to try and follow all the rules without just washing myself out, or, conversely, trying to make myself stand out without turning a potential employer off. Because you never know who is on the other end of that cover letter, especially when sent through an e-mail. You don't know their opinions on exclamation points and a little humor.
So crossing my fingers, toes and eyes that I can type with sincerity, and that whoever reads my cover letters can read with a little as well.