1) Some of the most popular contact form plugins, including Contact Form 7, don’t provide a thankyou page. That means that you can’t easily set up a simple ab-test in Google Website Optimizer (GWO) to increase conversions, because GWO requires a thankyou page. Doh! (I realize I’m running that plugin, but I’m not actively seeking new consulting clients, so it’s not a priority to fix that.)
2) Typically, the contact button says submit by default. It’s well known that you want to match the button to your headline, and people didn’t come to your contact page with servitude in mind and/or kinky S&M messaging. They came to contact you. So the default button should read “Contact Us” or “Send Contact Message” … Submit isn’t clear and cuts conversions.