Dude, come on. "it's pretty much a post-interview rejection letter"? Please take a zantac and a heineken and relax. I can think of about a 1000 things that are closer to a rejection letter. Think about the PD's perspective. The PD may have a strong interest in you, high enough to be ranked in the probable area to match, but definately not guaranteed to match. Or, they may not have met yet to rank you and have no idea where you stand. They may want to save face and be as neutral as possible rather than making you think you are guaranteed to match.
Furthermore, they may not have any idea who you are. After they read your email they may not have time to run and get your file, reread your lor's and personal statement, and review your interviewers comments and then take the time to spend all day thinking about which is the best way to get back to you to give you the most reassurance regarding your standing. Instead, a simple "thank you" is quick and polite.
Last time I checked, there is no
www.programdirector.net which has posts like "My favorite applicant sent me a letter saying they are going to rank me highly. What does this mean? He must not like me since he did not say 'rank #1' but then again why would he have written at all if he wasn't interested? In order to increase his chances of matching here should I email him back, write him a letter, send him a fax, call him, plan a second visit to his apartment, or tatoo his name on my butt and run naked through his medical school?" Remember, PD's have emails from tons of applicants, residents, faculty, etc in addition to their clinical responsibilities, research, family, etc. They're not living the fourth-year medical student dream of showing up late and leaving early from rounds, drinking a lot and exercising a little, video games, and logging on the nrmp site three times a day to constantly rearrange numbers 3 through 8 on your match list.
EDIT: by the way...you just sent them a letter raving about their program. I am sure the letter did not say, "I will rank you number 1 pending a reassuring response." They do not need to go on and on about how much they like you with a long response. They do not need to do anything else to increase your interest in the program. You've made it explicitly clear that they have already done enough to get your interest. She provided a simple "thank you" and moved on to the full inbox of emails from the hundred other eager applicants, many of which sent the same letter to their top ten programs, and responded similarly to them to get home by midnight.