I think the biggest thing of all time that grinds my gears like nobody's business...
...is the immediate hatred new gens and their Pokemon get because they're new.
Now, I'm not talking about people who legitimately dislike or even hate certain gens. And by "legitimately", I mean they saw the games, looked up some info on them, maybe even played them, and decided they didn't like them. They actually gave the thing a chance, even if only a small one, before they came to a decision.
I'm talking about the people who hear about a new game coming out and say: "Gen ___? Ugh, why are they making more Pokemon games? Anything after *insert region here* sucks. Original *however many Pokemon they consider to be good* forever!"
I will mention, I don't usually run into this type of fan on Serebii. I tend to see these people on facebook and it makes me want to tear my hair out. At least give the thing a chance before you tell me straight up that whatever group of Pokemon you like is far better than the new stuff that's coming out.