The Number One Mistake Interviewees Make Nowadays

I'm the Managing Editor of Business Insider, which means I'm responsible for all of the editorial hiring here.
So I'm constantly meeting people of all different levels, from interns to senior editors.
Lately, the majority of people I interview have one thing in common.
They're all messing up on something that I think is very important when trying to get a job: the Thank You Email.
Whether we spent thirty minutes meeting in the offices; we Skyped because you're abroad for your Junior spring semester; or we did a quick first-round phone interview, too many people are forgetting to follow up later that day or the next day with a quick email.
It doesn't have to be anything too involved. Truthfully, the shorter the better.

The Thank You Email should say a few simple things:
-Thank you for meeting (or talking) with me.
-I really want this job.
-Quick plug about why I'm perfect for it.

If I DON'T get a Thank You Email, here's what happens:
-I assume you don't want the job
-I think you're disorganized and forgot about following up
-There is a much higher shot I'll forget about you

Credit: Jessica Liebman (original title: "The Number One Mistake People I Interview Are Making These Days")