Oh, yeah, line breaks. No, I wasn't considering that a grammar issue, but I suppose it is. So you're right, posters often do get asked to 'fix' a solid wall of text post. And yes, the poster doing it can sometimes be terse about it.
That's really the only problem I have with it. The request is fine, I just think posters could be more careful with their tone.
As to the issue of another poster jumping in, I see it more as support for the first person to say something. Is one person just a crank? But if there's more general agreement, that's more information. In that thread, IIRC, Kai changed her style three times, from one unreadable format to another. So the posters still bringing it up were saying, 'still doesn't work for me'.
If they were saying "still doesn't work for me," I'd have less of an issue with it. The last post about it in the thread I saw wasn't anything like "sorry, I know you tried, but it's still hard to read." It wasn't even "sorry, I agree with Soandso, I'm still having trouble." The post really, honestly, seemed like it had been written immediately upon finding Kai's first post in that thread, before the poster even finished reading the other responses. It wasn't phrased as support, it was phrased as if it was the first post on that topic. Maybe this belongs in the "reading the thread" thread instead

As to ignoring the reasons, if someone has just said that they don't/won't/can't read a post because it's unreadable to them in that format, then there's no reason for them to know what that reason was, if it was explained to them in a post with the problematical formatting.
I'm a little confused by what you mean here. Are you saying that if Kai explained her reasoning in a post with centering, so it was still hard to read, then the correcter can't know what the reason is? Because frankly, it was a short post, so if you're going to go to the trouble of correcting a poster, and she's clearly responded to you (as you can see by the color change), I think you owe it to them to struggle through three lines of centered text, so you can respond, or hit the quote button so that little bit of text shows up in your reply screen and read it there. If it was a long post, or simply wasn't possible to read, I'd have more sympathy. At the very least, if you can't read it, you can probably deduce that if one thing changed and one thing didn't, the poster may have addressed that, and respond accordingly.
I'm not saying that people can't make these requests of each other (though I'm more inclined to just skip stuff I don't want to decipher), just that they should think carefully about how they're expressing themselves, and how unwelcoming they can seem, to someone new to the forum. It's important to keep in mind that lots of forums aren't as friendly as ehell, so "friendly request" may not be the default assumption a new user makes, when she sees a request telling her to change. All this post correction wasn't going on when I joined, and I'm glad it wasn't, because it may well have turned me off the forum altogether.
As far as I know, Kai is the only person on the forum who posts in something other than the default format. I think it's perfectly understandable that there was a bit of a bump in the road because of it.
Neysalmd, a long time poster, has been posting in turquoise for as long as I can remember. I don't recall anyone ever mentioning a problem with it, but I don't exactly follow her around the forum to check.