What's the consensus on fanfics that lean on or break the "fourth wall" where the characters in the story know they're in someone's fanfiction? Can it inject some humour into a light-hearted romp, or is it regarded as disrespectful?
I prefer to be fully immersed in a story, if I come across a blatant fourth wall break it's kinda off putting.
Um... well straight-up breaking the forth wall automatically makes your story a comedy. As far as I know though, most people are fine with that. If you're trying to write something serious, there are ways to sort of tap on the forth wall without breaking it. For example, in Kristen Beyer's book "Full Circle," Chakotay gives an angry speech about how unfairly and clumsily the Voyager crew has been treated by Starfleet since returning to Earth, and much of what he says sounds like it could just as well be directed at Pocket Books for its treatment of the "Voyager" characters in the Relaunch Novels. (This isn't an exact quote, but Chak says something to the effect of: "They use us when it's convenient....the information we discovered on our journey could have had everyone in Starfleet busy for eternity if they'd bothered to glance at it before filing it away and classifying it... and NONE of that compares to how Kathryn Janeway was treated, being sent to a Borg cube with only a science vessel..." Basically, Chakotay's speech against Starfleet seems to also be the author's speech against how previous writers didn't take "Voyager" serious at all, and used its characters as throw-away cameos for plot convenience or cheap drama.) Anyway, that's how you can break the forth wall without completely breaking it.
How about characters referring to common fanfiction tropes such as calling a character a "Mary Sue" or being glad they're not wearing red while on an away mission?
I'm all for having post-TOS characters joke about the "red shirt jinx" that their grandparents warned them about.
First-person is harder to write than you might think. You have to put yourself into the mind's eye of the main character / narrator. You must not allow yourself to "know" anything that character doesn't know. If the character wasn't there for watch something, unless he/she was told later, no matter how important it is to the story, you can't write about it, for the character wouldn't know to talk about it. I'm half-way into writing a story in third-person. I have a crew of 19 humans, but most of the story centers around one character, so she's in every scene. If she's not in the room, then I have selected one other character who will be. I have one such scene where she's not there but he is, written before I made this decision. Ergo, now that I have that choice, if I want two other characters to have a conversation behind her back, I have to have a reason for the secondary character to be in the room, too. Or I break my own rule. In first-person writing, you do not have the option, which means you may have to get creative how you introduce important plot elements to the story.