1. Fighting in the wrong place
This is one of the simplest errors people make and one of the ones you most often get called out on, as it's so very visible. In capture-the-base Battlegrounds such as Arathi Basin, Battle for Gilneas, Isle of Conquest or Eye of the Storm, fighting at the base will help you capture it. In Eye of the Storm, it's the only way to capture a base. You can't click the banner like in other Battlegrounds; rather, the game measures the amount of people within the base zone and captures faster or slower depending on that number.
In the other Battlegrounds I mentioned above, you click the flag in order to capture the base. But guess what? The other team aren't going to be super-keen on your doing that, so they're likely to try to kill you! Surprising, I know.
So what should you do? Fight them at the bases. Don't get pulled into conflict that isn't at the base if you can help it. Sure, you may be ambushed on a path by two of the opposite team's wandering rogues; you can't help that. See a lonely healer? Just don't, OK? I know, you're thinking, "Hey, a priest! I'll be able to kill them and I'll get another killing blow and more damage, and I'll go up the chart and get more points and just be generally awesomesauce!" Maybe you will -- but one on one, DPS vs. healer, unless there's a huge gear or skill disparity, it's going to take you a while. And in that time, you could have been helping your team capture or defend a base.
Flag capture Battlegrounds are a slightly different kettle of fish. I'm thinking Warsong Gulch here, and Twin Peaks, where people will no doubt get really miffed if you fight in the middle. But depending on the game, the middle isn't always a completely idiotic place to fight. If you can control the routes that people have to use to get the flag across the map, that's actually a good thing!
But if both teams have the other team's flag and are shored up in their own bases, having an e-peen off in the middle is not going to help. Your team's controlling the middle? Super! Bully for you. Now go get the darn flag.
2. Not defending
Speaking of e-peens, this is another brilliant way to fail at Battlegrounds by letting your ego run away with you. "Aha!" I hear you yell, "We have three bases here in Arathi Basin, so the Horde/Alliance must be weak and pathetic! I will lead my band of merry men and capture their home base and the fifth one! Total domination! Rawr!"
While you're off capturing, the other team calmly take all three of the bases you used to have. One lonely soul stayed behind and got insta-killed by five of the other team because their hero comrades were off capturing bases they didn't need in order to win. Oops. Or all your glaives or destroyers got immediately destroyed because nobody even tried to look after them. D'oh!
Now, I get it. Defense is boring, hanging out at a base hoping you'll get attacked is really dull. And it doesn't award you as much honor as capturing unless you do get attacked, which is a bit of a shame, really. But in Battlegrounds that are about capturing objectives, you win by holding bases. If there's nobody at bases to make sure the other team doesn't get them, you're not going to hold them and not going to win.
Of course, you can theoretically win Arathi by capturing all five bases and holding them, keeping a careful eye on assaults and capture timers, but it doesn't happen often, especially not in PuG groups without clear leadership.
3. Not attacking
However, not attacking isn't going to win you any prizes either. Obvious advice is obvious? Well, yes, yes it is, dear reader! But you'd be amazed how often (especially in Battle for Gilneas, possibly my favorite Battleground) there are five people sitting at the Lighthouse or the Mine while their team is taking a pasting at the Waterworks. Guess what: In order to hold bases, you need to capture them. And while you're all doing an admirable job of really spoiling that opposition rogue's day, five people at a held base is far too many while another one is in conflict!
And healers, I feel your pain. A good amount of the time, I'm one of you! Having a target on your head is really rubbish, especially once the enemy have figured out who you are and you become their prime target as soon as you appear. But my dear, sweet healers, your amazing skills are totally wasted in places where there aren't active conflicts. If people aren't taking damage, there's no healing to be done. You're not likely to be able to defend bases completely alone due to your inability to kill things -- and more importantly, you're far too valuable for that! There are probably only two or three of you to seven or eight DPSers, and you're worth the number of DPS's health bars that you can heal before you go OOM. Someone who's better at math than me should totally work that out.
4. Killing the wrong people
The wrong people? How on earth can you kill the wrong people? You can't exactly have friendly fire incidents in WoW, can you? Well, no, mercifully. But you can have almost as good as if you attack the enemy in the wrong order.
Now, please note that this assumes a fairly equal level of gear and skill across the two teams and is a generalization. OK? Great.
Go back to point #3 and re-read the last couple of sentences. No, not the one that points out how awful at math I am -- the one before that, the one where I mention how healers are awesome. How can you stop healers being awesome? By killing them, of course! Especially if there's only one healer in the area, they're not going to be able to survive a full-on onslaught from a group of DPS for long. Spot a healer, thanks to an addon like Healers Have To Die? Tell your team. (And if you don't have HHTD, get it.)
Don't go after that DK who's really not taking any damage; he's probably a blood DK and will outheal a healer. Leave him until last. When you arrive at a fight, assess your opponents. Healers, as we've mentioned, are highest priority. Tanks are lowest. People with low health are the priority right after healers. People whose name you recognize because they killed you earlier? If you kill their mates, they'll go down eventually. People who can CC like it's going out of fashion? Yeah, bump them up the list. People who are doing a great job of defending their own healers? They need to go down. People who were squishy before? Probably still squishy now. Your team will be a whole lot more effective if you play like this.
5. Being That Guy who QQs
In terms of irritation, for me, this is the most fail of all fail things you can do in a Battleground. It seems that as soon as a team starts losing (or even before then), there's somebody in chat saying "OMG you guys suck" or "L2P noobs" or "Alliance/Horde fail again" or something equally erudite and useful.
Do you think that's helping? Do you really? Well, it's not. How on earth is that going to make people play better? And all that time you're spending typing about how rubbish the rest of your team is would be far better spent fighting.
If you're frustrated, go do something else for a while. If you think your team's doing something specific wrong, tell them, but don't word it like "OMG, you idiots don't do thing X." All that's likely to do is annoy people, even if your advice is excellent and exactly what's needed. Phrase it differently, and people might listen and you might win.
Do you want to capture flags, invade cities, attack towers, and dominate the enemy for your faction? Do you dream of riding your War Bear with pride? We'll steer you to victory with secrets of Battlegrounds and Arena, prepping you with proven addons and keybindings that win! Send questions or comments to firstname.lastname@example.org.