Yeaaah Beekart ! Great insights here ! I especially like that one :

Motivate WHY you want to do work for them, be honest, be real... If you apply to a studio that makes mobile games, don't tell them you love all their games and think they are the best in the world OMG!.. They aren't a bunch of idiots. Be real!

For the reason we did it before them.... So... We know haha

It also really depends of what companies we are aiming for. Some care about polished illustrations or concepts while others might prefer more thumbnails. It all falls down as doing your researches first. Also as Beek said : Be realistic.
Artists in the industry can see if you have te required skills even with the thumbnails.

Give out a little bit of information about yourself.. Who is the company having contact with.
Again sounds logical but .. people just forget.

For this one I am wondering if it is not a misconception in the industry as everyone is stating that the portfolio is the only proof to get a job and nothing else matter, so people tend to forget the resume, but the resume tells a lot about that person and should be added as for any other jobs I guess ?

And last thing : Add how long it took you to create that piece, or at least know it in your head. Many AD's asked me that questions and it can be difficult to answer it if you don't track time.

Really cool article Beekart ! Thanks for sharing your knowledge ! All the little details are onpoint like to not do mass mailing, motivate why....

Cheers man ! (Sorry it was a long answer)