The proffered letter was actually pretty good to begin with. In some ways, that makes the suggestions for change even more instructive. Obvious errors like bad grammar or logical inconsistencies are easy to deal with, and could have perhaps been spotted in-house. But all writing can be improved, and in this case the commenters give some excellent suggestions for improvement, focusing on length (notice how long the original letter takes to get to the point?) and engaging personally with the reader.
For a quick writing lesson, take a look at the letter and imagine how you'd rewrite it, then read the comments that follow. Learning to edit is a great way to learn how to write!