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Three things:
1. Using the word "gay" and "faggot" as synonyms for bad. Homophobia like that is becoming unacceptable out in the real world, and the internet should be subject to the same rules. Also, using excuses like "Geez, it's just the internet!" when they get called on it. My rule of thumb: Never say anything online that you wouldn't say out loud to a room full of strangers in a public forum, or it will bite you in the ass someday.
2. Obnoxious, clashing layouts that do stuff like put lime green text on pink backgrounds. My eyes people, you're killing them! People also need to learn how to code properly and not keep messing up pages (I'm looking at you, TVTropes Wiki!).
3. Anonymous vandalism to websites that is blatantly untrue. At least once a week I end up having to revert a page on Wikipedia (yes I edit it, mostly cleaning up other people's messes because I'm a masochist) because some idiot thinks that by changing it to read that Tohru ends up with Yuki in Fruits Basket it'll become true. That happens far too often for comfort.