Be the James Bond of queries. You assume your capability will be self-evident. That's just as impressive on paper as it is on screen. Insecurity is also revealed when you don't know what your script is about and its genre. If you have to take three or more sentences to describe your script, it shows that you aren't confident that you know what it's about either and that really really bugs editors, agents, and producers.
Insecurity is revealed by verbosity, excessive adjectives and weak verbs.
The shorter, stronger and vivid a letter is, the more it strikes us as interesting. Plus, it's just bad writing as it tends to bury the lead. The best part is stuck in there with so much other stuff that they miss it entirely. As an exercise, invent a character who is going to write a query letter. Make the character confident, capable, poised and give her a certain flair.