I wish people would grasp the concept of not initiating a team fight when you are out numbered. I dont know how many times people just jump into a 3v5, die, and get mad about it. If you're outnumbered and not severely fed, dont fight. Fall back to a tower, hold them till your other teammates arrive. Or better yet, keep them busy while your teammates push lanes.
I also keep seeing this situation. One player gets caught in the jungle, but is running back to base with low health the other team in pursuit. Maybe one or two of his teammates show up to help, but he's too injured to do much. Instead of just escorting the person to a tower, people will initiate the entire other team. Lets do the math now, theres like 3 of you, 1 almost dead, and your other 2 are on their way but not here yet. You're up against 5 enemies who are most likely all full health. How is this going to turn out well? Everytime is ends up with all 3 of them dying, and the other two showing up too late. Usually one or two of the enemies are low now, so at least one of the remaining teammates will think "I can get them!" ignoring the fact that there are still 3 almost full health enemies with them. Then you end up with one guy left to defend against 4-5 enemies while everyone else is dead.
Basically people dont understand you just DO NOT fight when you're in an equal or better position than the enemy.