It's just the undertones. It's like "I expect you to do this for me." I used to interpret it as, "You're so friendly that I know I can count on you" but in a work environment (or mine at least), it seems more manipulative than that.
It's a great phrase! It is passive aggressive and strong-armed, yet completely polite and acceptable in the business culture. There is no better way to force someone to answer your e-mail than ending it by thanking them in advance for their help and response.