Engineering hiring managers, what do you consider a good cover letter?

Here's my opinion from decades of hiring. First, I think cover letters are extremely helpful and should always be included. They should also be relatively short. Certainly less than a page, ideally just a couple of paragraphs.

From the hiring side of the equation, the purpose of the cover letter is, basically, the sales pitch. Your resume tells your work history, but doesn't tell anything about why you're applying for this specific position at this specific company.

The cover letter bridges that gap. Ideally, the applicant explains why the company and position are interesting to them, clarifies how they expect to use their experience in the position, and provides a way to explain strengths the applicant may have that are both specially relevant to the position and not obvious from the resume.

Also, if there are things in the resume that may be less than flattering (work history gaps, apparently unrelated work experiences, etc.), the cover letter is where the applicant can tie up those loose ends, or explain possible misinterpretations.

And also very important is that the cover letter demonstrates that you know at least something about the company you're applying to. You don't want your application to look like it was just randomly shotgunned out.