Most of these fall into the "standard," boring category. In other words, these emails rip the style and content of what the university career center tells them to do. Toolish, impersonal writing will not leave a memorable impression in your favor.
Thank you emails are extremely low risk but have the potential to be high reward.
If you hit it off with the interviewer, and mention something personal you took away from your conversation, he or she will remember you. At the end of a interviewing day, when the banker/trader/consultant has seen at least 20 rehearsed applicants, it's hard to stand out. And the interviewer knows a good chunk of the interviewees were not that interested in the first place.
Beyond including a personal shout out, we've helped students in the past who used the thank you email to keep in touch with their interviewers, regardless of whether they advanced or not.
If the interviewer just didn't like you, it's not worth staying in touch or trying to convert your conversation into a networking opportunity.
But I'd say that 80% of the applicants interviewed just didn't have enough industry, inside knowledge to land the job, and that with more practice and knowledge they landed positions for full time. Part of that recruiting success comes from knowing people who will go to bat for you down the line. Some people are willing to mentor or give advice to young people more than others!
So, what should a great thank you email look like?
First, keep it short - no more than 5-6 sentences. No one will read beyond that.
Don't use flowerly language or be over the top with positive words describing the firm, especially since everyone does that and it makes you sound insincere. Consider the 1 or 2 things that YOU found unique to the firm/interviewer, and highlight those.
Contain a short reminder of what you talked about during the interview, which gives the other person the opportunity to recall what you said and his/her impressions of you.
Last, include a closing line that shows you're interest in the interviewer -- that you'd be sure to let them know of what happens, etc. If the interview was extremely personal and all-fit, and you both had a shared interest, maybe include an interesting article link or something to that effect. That always runs well!