Don't Play Ranked If You (sometimes even normals depending on the severity)....
Easily Tilt When you go on a tilt, you start yolo'ing everything. You don't ward, you don't try, and you just make the game into a 4v6, making the game almost impossible for your team to win because of you. Please don't play ranked if you easily tilt so you do not ruin others' games.
Are Extremely Unskilled Ok, I didn't really want to get to this topic, but I have to address this issue. You can ALWAYS play norms to get better, and I'm not talking about 'bad-below average' people; I'm talking about people who are reaaaallly unskilled. No offense, but you ruin people's games if you are very unskilled (both in norms and ranked, but especially in ranked). As i said, you can stick to normals because really no one can say anything because you are trying to learn. And if you are not ready for norms, you can play bots :D
Do Not Communicate Well This happens mostly in ranked where a player just refuses to communicate in-game or during champ select. They would either refuse to give up a role or just lock in whatever they want to play. Chances are, you will not win the game because you refused to communicate.
In Closing
You can obviously do w/e you want because I really can't make you do anything. But, it'd be really nice if most people who play league were more considerate and had better attitudes.
As always, Have Fun and Good Luck :D