Back when I edited a daily newspaper, I hired reporters, paginators, editors, and the occasional photographer. Any job posting resulted in a flood of resumes, but many of them were moved into the "no" pile immediately.
In most cases, the easiest rejections weren't the people who were totally unqualified. In fact, some of those used their cover letters to make a viable case for getting at least a phone interview. The easiest ones to reject were people who made these mistakes.
1. No cover letter/generic cover letter
Some big companies sort resumes via automated tools, making a cover letter obsolete. That means you can skip writing when the application process leaves no place to attach one. In every other case, however, it's important to have one.
And, just having a cover letter isn't enough. You need to craft a cover letter that shows you're applying for this specific job. Address something in the job ad and connect your skills as directly to the position as possible. If there's something being asked for that your resume does not clearly show you as having, explain how you can meet that need (or explain how you will gain that skill).
2. An absurd or conflicting objective
As I've covered in the past, I'm generally against the concept of putting an objective on a resume. Doing so almost never adds anything, and even a well-crafted one comes off as contrived in most cases.
Having an objective won't get your resume put on the scrap heap if it makes sense. If I'm hiring a reporter and your objective is "to use the skills I learned at my college newspaper to land an entry-level reporting job," that makes sense and is not off-putting.
In many cases, though, objectives are either too generic -- "to secure a position at a firm that can use my skills" -- or actually conflicting. If you're applying to be a night watchman and your objective is "to land a job where I can be outside during the day," well, then you're not doing your candidacy any favors.
3. Big unexplained gaps
If I look at a resume and there's a large gap in employment that's not explained, it's a major red flag. Maybe you left the workforce for school, maybe you left for family reasons, or perhaps you went to prison for some terrible crime.
You don't need to be incredibly specific. It's OK to explain a gap in vague terms (unless it is for something like a prison term that will be discovered on a background check). In general, it's OK to be a bit vague: "I was out of the workforce for three years dealing with a health-related issue." That could be cancer, a sick parent, or rehab, but it's at least an explanation. Not offering at least some reasoning for a gap makes it seem like you're trying to hide something.
Use every tool
Think of your resume and cover letter as a coming attraction trying to get the person doing the hiring to want to see the feature (you). Make every word in each one count.
A cover letter is an opportunity to show your personality and make a case for yourself beyond the raw accomplishments shown on your resume. Your resume is a chance to show not just dates and job titles but hard accomplishments you achieved in those jobs.
Take the time to make each application personal and relevant. Avoid the temptation to just apply for a lot of jobs and hope you get noticed. Quality is better than quantity, and spending more time fine-tuning should result in more interviews.