1. If something would be boring and/or undramatic for a male character, it would probably be boring and/or undramatic for a female character. If you’re writing a female character (particularly in a major role), I’d recommend thinking about whether you’d want to read about a male character in that situation or with that trait. If not, then you’re probably boring your readers.
2. The character is useless. Have you made a main character more or less helpless for most of the story? Does she watch as the story happens around her? Does she get repeatedly saved by other characters when the going gets rough? Please think back to #1. You’d probably be bored reading about a more or less helpless guy, right? Your readers will be just as bored by a helpless female.
3. The character’s only defining trait is being hyper-smart or (more rarely) a total ditz. That’s fine for one character among several, but if she’s your only significant female character, it’ll raise questions about your ability to handle female characters at a more relatable level of intelligence.
3.1. The character is totally pure. A character that always does the right thing and has no motivations besides being friendly/agreeable/nice is probably pretty boring. 100% pure characters strain the suspension of disbelief, are less relatable and usually less dramatic. For whatever reason, these types of boring characters are almost always women.
4. Your readers will probably be able to tell if you have not read many female main characters written by female authors. If you don’t have the firsthand experience of actually being a female, being well-read is probably the closest you’ll get to seeing the subtle distinctions between most women and most men in terms of perspective, dialogue and actions. Conversely, when I’m reading manuscripts, the easiest way for me to pick out male characters written by female authors is when 1) the character is hyper-introspective and collected (even in a crisis) and the author doesn’t realize that’s unusual, and/or 2) a male character notices far too many irrelevant details, such as eye color and hair color, and the author inadvertently makes it sound like the character’s ogling someone or writing a fashion review.
5. The character is a love interest that doesn’t have a role outside of romance. She’ll probably be a more interesting love interest if she has something else going on. For example, Lois Lane is (occasionally) a competent reporter whose investigations sometimes tie into Superman’s work. Pepper Potts figured out who kidnapped Tony Stark by breaking into Stane’s office. Ramona Flowers from Scott Pilgrim had a penchant for awesomeness and a mallet. Also, she was a ninja courier for Amazon.
5.1. The character is defined by her physical attractiveness and/or sex appeal. If you consider physical attractiveness one of the three most interesting things about a major character, I would recommend rethinking the character’s development because most likely the character is a love interest that is interesting only to the author. (Think back to #1–you wouldn’t want to read about a guy whose main trait was his handsomeness, would you?) Also, please bear in mind that most of the professionals evaluating your submission will probably be ladies, so you won’t even have the titillation angle working in your favor.
6. The character has no substantial goals besides going along with other characters and/or getting in bed with somebody. If you’re going to bother writing in a character, I’d recommend giving him/her some sort of independent effect on the plot. If not, why bother having the character? Fortunately, you don’t need to give a character much space to give her/him a role to play. For example, Neville Longbottom had around a page of dialogue (~350 words) in the first Harry Potter book and he still managed to raise the stakes for the protagonists by growing a spine at absolutely the worst moment. (Dumbledore’s recognition of his badassery was probably the highlight of the first book for me).
7. The character is mute. In general, I think the mindset behind this decision is “I’m having a lot of trouble writing dialogue for females, so I’ll just make her mute.” In this case, muting a major female character will only draw attention to how bad you think your female dialogue is. I’d strongly recommend practicing your female dialogue instead–the practice will help, and at least you’ll get out of instant-rejection territory.
Must read for anyone who likes writing things or likes criticizing things, or really just anyone. Particularly male writers, but really, everyone.