Everyone has played with someone that they thought was horrible. On my server (and probably on most), these people are know as "bads". You will see people in LFG saying, "LFM for Emalon, no bads please". How does someone know that they are bad?
I think that the people who are bad are probably oblivious to the fact that they arent very good. Not to say that these people are dumb. They are usually just lacking something. Sometimes they are dumb. Or little kids. But that is a whole different issue. Here are some ways to tell if you (or someone else) is a bad. One of these does not make a bad. All of them together most likely will.
1) You dont have addons or dont even know what addons are: This problem is very easily fixable through a little bit of education. Addons are a core part of the game at this point and can do just about anything for you. If you dont use Auctioneer, you are a financial "bad". You probably vendor everything rather than looking at the AH or you put things up for such a low price that you are kicking yourself in the ass. There are plenty of bads who do have addons. Bads with the correct addons are just plain bad. They know better.
2) You gear completly wrong or using dumb methods: Bads that make this mistake are often lacking the knowledge to make it right. A hunter with spellpower gear, a warrior with spirit gear, and the person who prefers their items to match rather than be effective are all bads. Wearing armor that is "outside" your class, does not make a dumb. If cloth casting gear is an upgrade for a Paladin, they are not "bad" for taking it. They are actually smart. People that only gear based on armor class are "bads" AKA the Paladin who will let a piece of mail get sharded because it is not plate despite it being a clear upgrade. By the same token, tanks that wear leather (besides Druids), mail, or cloth are epic fail.
3) You don't understand basic game mechanics (or your skills and abilities): If you have no idea what someone is talking about when they talk about aggro, dps, or CC, then you may infact be a "bad". If you do not know a what a damage rotation is, you may be a bad. If you use taunt as part of your damage rotation as dps, or keep growl on no matter how many times people tell you not to, you are a bad. All of these things can be learned. All it takes is a bit of motivation. If you never figured out how to put your abilities on action bars, and cast out of your spellbook, you are a bad (how did you even install the game?).
4) You can't seem to help it when you stand in the bad stuff: You go into every fight just knowing you are going to die. When you do die, you make no effort to figure out why, continuing the cycle of bad. This can be the Heigan dance or riding a drake in EoE. You figure that despite your shortcomings, the rest should be able to pick up your slack. This make you not only a "bad", but dead weight. People resent you. Get out of out raids and go run stockades or something. Everyone dies to the bad stuff sometimes. Just...not....every....single...time. There are legit excuses for dying to bad stuff. Being from Australian and playing on a US server is one. Because "you always die on this fight" is not an acceptable excuse.
5) YOU ARE NOT PREPARED: Illidan apprently knew that there are tons of baddies around. You show up for instances/raids with broken gear, no reagents, no ammo, no buff food, unenchanted, and ungemmed gear. I did a VoA 25 the other night where a "bad" enhancement shaman got kicked. He was doing 1200 dps and wearing 41 stamina gems in his gear......WTF? There are reasons to not have SOME enchantments. If you are going to be replacing an item shortly or are leveling are good reasons. There is never a reason to have gear with no gems. Green quality gems are so damn cheap that there is really no excuse.
6) You get kicked from groups...repeatedly: If you are contantly getting booted from groups, it is not the group that failed....look in the mirror...it is you. When people do an instance or raid, they expect a certain level of competence. You face rolling your way to epics with 800 dps is not going to make you any friends. Pulling groups if you are not the tank will not make you friends. Mis-directing on the healer will not make you friends. They will all get you kicked. Because you are bad. Do yourself a favor and figure out how not to be bad.
Obviously, this list is not going to help the bads. This is a list for the rest of you to show the bads to the light. There are so many resources available to up your game. If you are too lazy to look at some of them, I can only imagine what you are like in real life. I hereby declare June 1, NATIONAL HELP A BAD DAY.
If you know a bad in WoW, this is a day to try and help them out. Instead of immediately booting the bad, try to help them out. If they are not receptive, they are habitually bad and dont deserve your time. If they hear what you have to say, you may well be on your way to rehabilitating a bad into a decent player.