At @dantesolar 's suggestion, this is getting its own thread.
Oh man this could be a long list LOL I used to be so bad.
First and foremost, PATIENCE.
Never rush in if you don’t have an escape, enough survivability, and / or appropriate back up.
Play Melee characters carefully and choose your battles wisely, do not get impatient and try to rush in, especially against ranged-heavy teams.
Always stick with your team unless you absolutely have a reason not to (ie, a valid chance to flank, encircle or Backdoor, but even then, regroup ASAP).
IF YOU ARE GALILEa OR ISIC OR BOLDUR, REMEMBER TO USE YOUR DAMN SHIELD!!!
Don’t peel off to get Thralls at bad times or if you don’t need them. Stay with your team.
As ERNEST or Kleese or Ambra or even Miko for his ultimate, choose safe, hard for the enemy to reach spots for your Eggs / Rifts / sunspots / Mushroom head.
DO NOT BUY THE CENTER INCURSION LIGHTNING TURRET UNLESS YOU CAN DEFEND IT. Also, do not destroy it just for the sake of destroying it if you are not even in the area it threatens, this will just let the enemy rebuy it and feed on the experience.
If you find you are dying a lot, hang back. Gather shards, build stuff, grab Thralls. Just don’t rush in to get killed again to feed the enemy.
Use a cheap or free shard generator (if it’s a free one, make sure the negative ability on it does not affect your character).
If you are on mic with people, even if you are the only one on mic, try to call out warnings, when you see weak enemies, when you are going to try to stun someone, when you need covering fire for retreat or distracting fire for a sneak attack, etc.
Remember that back Thralls respawn every 2 minutes (28, 26, 24, etc) and that middle double Thralls respawn every 5 minutes (25, 20, 15, 10, 5).
Last but not least - IF YOU ARE LOW ON HEALTH, FIND A SAFE SPOT AND PRESS “DOWN” ON YOUR DIRECTIONAL PAD TO TELEPORT BACK TO BASE!!!
Hope this was helpful!