I always end cover letters with, "Thank you in advance for your consideration." It politely gives the recipient a call to action.

Hmmm...if the comment was that you talked too much about how wonderful the company is and why YOU want to work for THEM (which seems to be indicated by "too much of a suck-up"), then perhaps you shoud focus more on why THEY should want to give YOU the job. The cover letter is the place to talk yourself up - to point out the skills and experience that make you perfect for their company.

My cover letter attitude:

Your time is valuable, so I don't want to take much of it. But you really should take a moment to look at my resume and see how valuable my skills are to your organization. I didn't just pick you out of a pile -- there are specific aspects of your work and mine that brought me to you. We should meet and talk soon about what things I can take care of immediately with my present skill set, and also what training or "professionalization" your company can offer to our mutual benefit. Thanks for your attention/consideration/time.

I usually keep it brief.

Use paragraphs.

I've never spent that much time reading cover letters. Keep it short, professional and friendly. The more you say, the more chance there is you're going to blow it.

If you got that much honest feedback maybe ask for a little more: what exactly made you letter too suck-upy? You are a good enough writer to know what to correct if you can get that extra bit of info that seems to be stumping you, and though it might be too late for that job it might help to improve your next letter and your chances for interviews in the future.

As far as general construction of a covering letter goes, that is really hard to define in my experience. It depends on how much you know about the job and how much competition you know it will generate. If you know a lot and there is little competition you can afford to be more loquacious (no, not that loquacious). If the competition is intense you have to be a bit more tabloid-y to catch a screeners eye.

My general rules are:

1) Write clearly, consicely and explain my qualifications in a direct and intense way--even point form if I think it might work for a certain job, but if so, do it almost off the top (and all of that spun as needed and relatively truthfully depending on the nature of the posted "job description." See 3.).

2) Balance 3/4 parts knowlege and experience with 1/4 part enthusiam for the position I am hoping to interview for. Base applying for this position primarily on my past accomplishments yet exhibit an honest willingness to learn/retrain as needed.

3) Read the posted job descreption many times depending on how much you think you want it. My knowledge of the job/company should be evident in my explainaition of my qualifications and abilities and therin lies the spin: they know what they are looking for so I need to show that I can already do it based on my past experience, but that I am "teachable". Everybody wants to keep training to a minimum so if they see my past employment as almost fulfilling the position they need to fill then I am golden if I am humble.

4) The obvious: spellcheck. ;-)