1. Speaking in leet speech:
"1 4M TEH M@$t0R 0pH 4lL OF j0O pH33r MY 1337nE$5 H@H4HH@"
Contrary to the name's implication, leet speech does not make you "leet." It's a stupid language that only impedes your meaning, which is usually something along the lines of "I rule, you suck". Face it, if you find that you MUST type in this language to make yourself stand out, you probably shouldn't be living.
2. Lack of punctuation, capitalization, and proper spelling:
"hey does anyoen kno were i can find teh spriet sheet of mario from smw i need it for my new comix thats gunna b super cool k well if anyone has it than can you send it to me k thx bai"
At the part where you say "teh spriet sheet," most oldbies will immediately pass you off as a brainless n00b, and will likely not respect you. I know it's a little hard to reach your pinky over to the Shift key and/or the punctuation keys, but if you put a little effort into making your posts legible, it'll pay out in your reputation.
Furthermore, typing in all caps, like this: "HEY WHY WONT ANYONE GIVE ME THA MAROI SHEET I NEED!!!!!! YOUR ALL MEAN!!!!!!!!!!!!!!111111" usually followed by seven or eight angry posticons, has a similar effect to having no caps, except it pisses people off more. Believe me, n00bs: typing in all caps does get people's attention, but in a very, very bad way.
3. Overusage of netspeak:
"k im sorry for makeing u mad but i really need the sprite sheet 4 a comix that btw will b way more 1337 than ur comix lol well i g2g k so l8r"
This usually ties in with #2. "Netspeak," the usage of numbers and acronyms in place of real words to make quicker messages, is suitable only for IM conversations. Most board moderators don't tolerate the overusage of netspeak. Others will either ignore you or belittle you.
4. Threats of hacks, viruses, flames, etc.:
"If you don't give me my sprite sheet, I'll hack into your computer and sell all your personal information to theives! Or maybe I'll just make your life on this board a living hell."
First of all, no one will believe you. No one. At all. Mostly because hackers usually carry through their activities in secret. No intelligent hacker will announce it to the world. Second of all, hacking threats are incredibly annoying, and common. The same goes for viruses. Thirdly, flame threats are just lame and pathetic. If you're gonna flame someone, just do it. Of course, you'll probaby get banned... but how long did you think you were going to last the way you act?
5. Self-promotion, or excessive bragging:
"I beat Super Metroid in 3 minutes, with 100% completion and without getting hit once. I rule."
Sorry to disappoint you, n00b, but you don't rule anything. Nobody does, and anyone who says they do is an attention seeking loser. Believe me, people know you are. Just by the fact that you waste your life away making pointless records with video games people stopped playing ten years ago. No one's going to try to beat you. No one wants to. No one cares. Instead of finding ways to promote themselves by bragging about their useless achievements, most normal people spend their time playing games for fun. And by "normal", I mean the average internet user, whom the culture at large has labelled a loser. Guess what, you self-promoting little n00b? You're considered a loser to losers. Congratulations.
6. Bossing others around:
"Hey, don't quote him! I don't like him, and you shouldn't either. Oh, by the way, can I be mod?"
Asking to be mod is like asking to be ignored. If a board needs a new mod, the old mod will post a "Mod Wanted" topic. And it's always the people who are liked that get picked mod. If you tell people what to do, then you can be damn sure that people don't like you.
7. Bold, provocative claims, and/or being a fanboy:
"Why do you guys talk about Mario? Mario sucks. He's so gay. So is Gamecube. Xbox rules above all! Play Halo, you gay fags!"
If you join a video game forum just to provoke people, go into your garage, get a case of antifreeze, and guzzle it until you're dizzy. You're a dumbass if you have an unwaivering devotion to a single console. All consoles have their ups and downs, and so do their games. Being a fanboy is idiotic, so don't do it.
That's about it. If you do any of the above, then I'm sure there's an elementary school teacher near you who will be glad to increase your knowledge.