I basicly start ranting if I notice one of a few things, these can, and have, been issues I've met within the Mabinogi community and elsewhere in life. Not fully game related though.
Being mean in general. Everyone is human, and being a jerk can always be avoided. Don't be a Renegade, be a Paragon.
People starting to hit on religion. I'm not religious myself, but it usually causes those who are much distress. I prefer open and friendly discussion over beliefs rather than bashing of them.
Racism, as mentioned above. It's a global issue, and it's even sadly present in Mabinogi. I don't think I have to remind you of some of the people in the game who openly attack those who speak other languages or tell others that they are not american or english. Though getting rarer and rarer, it's still an issue, sadly.
Homophobia. Not that I'm gay or anything, but I've befriended quite a few bisexuals and homosexuals. There is literally nothing wrong with these people, and hating on them because of their intimate preferences is just silly.
And for the final less serious one,
Gunship armadas in Supreme Commander. The lack of balance is real.
Probably drifted away from the general flow of the topic here, but wanted to give my voice here too