First things first, attitude. People don't respond to rage well, and although it's incredibly tempting to cuss out that one guy who keeps going far and dying, or that other guy whos camping close like it's got a black Friday sale going on, rage never ends well. Try suggesting in /t "Hey, <x>, would you be able to back mid up? Close is safe." of course, it's all situational.
Secondly;** rotations**. over-committing to a point is a thing I often see in my games and in general. Judge the fights accordingly, can my teammate successfully kill the enemy? Does he need a +1? If he needs a +1, go assist, and if one of your teammates comes also, break off and rotate somewhere else. One guy doesn't need 3 people to kill him, and if he does, you're better off rotating to another point anyway. The same for losing fights, ask yourself, are we going to lose this fight? If the answer is imminent yes, and you can break off, do it. Break off and rotate to the point that is free because most of the time there will be the enemy team at the fight you just lost. If you can't decap said node, move back to your capped node and wait for your team so you may regroup and go back in strong. Trickling is a game loser more often than not.
I think another point would be map awareness, which is a skill that develops over time but goes hand in hand with rotating. Take a note of which classes are where, and who is fighting who. This will give your a rough idea of where you are needed for your current professions role. Is there a thief that was spotted on the map moving in the direction of your close point? Odds are he's going for a decap, move to intercept. The enemy elementalist and druid are at the far node, fighting two of your teammates? This isnt a fight you'll win, but more often than not it means the contested node is going to be increadibly weak for them, ask your team to break off and go there instead. Call out in /t if you need help, or ping on the map.
Cleaving/Stomping and ressing. Make sure you cleave downed bodies if your profession allows it! That one person could make the difference between winning the fight and losing the fight. Poison, daze, stun, do whatever you need to do in order to keep that body down. Ressing your teammates; know when to accept it's a lost cause, too many times have I managed to secure a double or triple kill due to the enemy over committing to the res, if there is poison/revenant cleave/condi pressure/stuns rolling over that body, unless you have a sufficient amount of teammates ressing that one person, it's probably a lost cause.
Any other tips /r/guildwars2 might have for new-er and even more experienced PvPers is welcome in the comments. Thank you for reading and I hope this helps at least some people.