Ask anyone what makes news and you'll likely get a variety of responses: timeliness, impact on one's community, matters of public safety and even the odd or quirky story can all be judged to be newsworthy.
In reality, it's a matter of opinion. With the exception of a blockbuster story, what's newsworthy to one person may not be to someone else. What captures the attention of one editor may get a yawn from another.
That's actually good news. Why? Because reporters, editors and even bloggers don't all think alike. Even if only one of them is interested in your press release, that's one more than before you sent it. So, don't assume that a rejection from one or two editors means your story won't generate any coverage.
Conclusion: learn to think in terms of what's newsworthy, what can help a reporter do his or her job. But never forget that it's a subjective question. No two editors or reporters see things exactly the same way.
Use that to your advantage.