He literally said that he detested it, which seems to transgress normal bounds of dislike, and then accused it of being adolescent, goofy, unoriginal, and dull for reasons that seem that do not seem particularly well grounded.
Matters of like and dislike or detesting something do not have to be well-grounded. It's a matter of subjectivity. Not objectivity. Somethings just rub people the wrong way.
For example, I really like the Eberron Setting. But I just can't get past the incredibly "stupid" time line. Age of Demons 1,000,000 years? Age of Giants 80,000 years ago? Etc., Etc. It just totally took the setting and kicked it right in the teeth for me. Even a race that lives for a 1000 years gonna have a hell of a time remembering what happened 1,000,000 years ago. And the Orcs are the guardians against the demon incursion. A race that lives 80 years? Really? Hell, they don't even appear in the time line until 30,000 years ago. 70,000 YEARS after the end of the age of demons. AAAaaaargh!
Others have explained it, but it still just ruins the whole thing for me.
I think that it's fine to dislike Eberron.
But not if they give reasons you don't like.
It's not to everyone's taste. The aesthetic can be off-putting. Some may prefer other setting elements more. But you can dislike these things without engaging in typical name-calling behavior. Imagine, for example, if he had simply said, "I dislike halflings riding on dinosaurs." That's fine to me. However, saying the setting seems adolescent because it features halflings on dinosaurs and saying that it seems like something a 14-year-old would come up crosses the line.
No. No, it doesn't. It's his opinion. Is it a tad insulting? Yup. But, in my opinion, he can state it however he likes as long as he doesn't break the forum rules. The fact that you don't like the way he said it and it hurt your feelings for some reason in no way "crosses the line."
From the bleachers, you're the one who looks like the one who is crossing the line by attacking him for not sugar-coating his opinion that he was ASKED FOR.
If you reeeeally think he went over the line, why don't you just report him instead of attacking him?