It all started with a question.
Stefan steps up with the most basic advice that every recruiter should understand. We are not email addresses, Twitter handles, or Facebook profiles, we are humans. Treat us as such. (The fact that this was the very first response should tell you something.)
Harassment is emailing us over and over. If you email us and we don't respond, and we don't respond to your second email reminding us you sent us the first email, YES, we are ignoring you. Move on. Don't email us again asking us why we aren't responding, and please God, don't get snarky. Remember, we talk to each other.
Give us the relevant details. 1) We may have already applied to yor client. 2) We might not be able to afford to take the job, no matter how awesome it is.
Joe hit the nail on the head. Don't resort to hyperbole, don't overinflate the opportunity. Be genuine with us and we will return the favor.
..and now for something completely different. Here is one from the other side of the table.
Many recruiters think of developers as the product they are selling. Stop that. Remember, we have choices, treat us like a product that needs to be packaged for your client and we may decide you aren't the right choice for us.
Seriously, if we did Java at one job, 10 years ago, don't send us "Awesome Java opportunities that you are perfect for" emails. Just stop.
Buzzwords make you sound clueless. We don't like dealing with clueless.