Keep the cover letter brief.
For a themeless puzzle, almost nothing needs to go between the "Please consider this puzzle" line and the "Thanks for your consideration" line. I sometimes mention the word-count and the targeted day (clue difficulty level) in that first line.
For themed puzzles, it will all depend on the complexity of the theme. If it's a standard type, then one or two lines should suffice. For example, "This puzzle's theme is an interpretation of the phrase XXX YYYY XXX -- located at 21-across. The remaining theme entries are phrases built from synonyms of YYYY." Also, mention anything out of the ordinary: "A helper entry ZZZ is located at 57-across" or "The puzzle may require the following notepad explanation which cannot be easily embedded in the theme clues: ...". Just spell out all critical elements with as few words as possible.
Do not point out weaknesses with the fill. First the editor may not agree with you; Second, he may find other weaknesses that you didn't know about; Third, he is very adept at locating those weaknesses himself; Fourth, he may easily adjust a small segment of fill to fix those problems; Fifth, he'll judge the puzzle mainly on the theme -- and request a new fill if it's not fixable.
Empty puzzle grid
Completed puzzle grid
List of clues (and entries) ... check layout requirement.
Since your name and contact info is on the attachments (and letter header) you needn't mention anything about yourself in the body of the letter. After the editor contacts you to accept your puzzle, the payroll people will contact you for their details.
That's about the gist of it.