The thing I see wrong with a lot of players is they think that they are so much better than they really are. THey think that just because they have a great score that they did something great. There are a lot of factors in LoL that low elo players don't grasp and claim that the system is cheating them and keeping them down.
First thing I see is the CS problem. Low level players don't put enough emphasis into CSing and focus on simple kills. While its not arguable that kills give you leads, but when you watch high elo matches in the LCS or just soloque you see players bullying the opponent off the minion wave and freezing the lane so they lose cs. TO keep it simple 15 cs roughly equal 1 kill. SO if you are up 15 cs (roughly 250 gold) that means you are also up roughly 1 kill in gold. You can also look at it as even if you get 1 kill, but you fail to cs and your opponent has at least 15 more than you then you actually don't have a gold advantage on them. You are equal in gold and equally as strong. When you start getting higher having 1 extra pot can mean a won or lost lane.
Next thing is warding. Keeping not only many wards on the map from EVERYONE, but good wards. Places that let you see exactly where people are going. MIA's are nice, but if you are properly warding there is no need for MIA's. You will see the enemy coming. The only role I don't buy wards as is ADC unless I have a decent lead on my opponent and then I'm always getting pink wards to get the most out of my money. I see it in gold where people still think that in season 4 that only the support needs to get wards out, but its just not plausible anymore. Everyone on the team needs to ward.
Objective control. I'm still seeing it now. People chase for days for kills instead of taking down 1 tower and opening up the map more. When you get a good advantage you need to use it. Going back to lanes and farming is only helping the enemy team if you are ahead. When you kill their jungler look for dragons/barons/buffs to take from the map. WHen you see 2 enemies top pushing an outer tower and your whole team is mid push hard on the tower. That trade is a great trade and it opens up the whole map and makes your opponents play more defensively. I've seen numerous times where players would rather save 1 outer tower instead of going for a mid inner tower just to not show up in time. You need to be focused on the goal of killing the nexus and not with getting the most kills or saving the most towers. You will lose towers and its an acceptable trade when you are gaining something in return.
Ending the game is so hard at lower elos. I remember many games going 45+ minutes because we didn't understand how to end the game. You need to know when you are able to close it out and how. The longer the game goes the more even teams become.
Use your teams strengths for you. I've had games where we have champs like lucian, leona, Amumu, Orianna, and gnar, but the team refuses to teamfight. Thats ridiculous. Use your strengths for you and not against you. That teamcomp should not be split pushing, but forcing fights post 6. If you have a champ like
/
/
/etc you can focus more on split pushing and just stalling in other lanes, but make sure you have a solid 4v4 or you will lose. Don't pick 4 squishy champs or your opponent will beat you.
Know your champ. I can't stress this enough. I recently played with a guy who said he knows how to play
. Well to put it simply he lied. He didn't know how to build him and he didn't know what half of his spells did. That ruined what could have been a great pick into a huge hindrance. Regardless of whether its a meta pick of not you need to know how to actually play/build your champ. You need to know when to combo abilities/trade/roam etc. I main
and a key part of knowing him is knowing when to flay. I've seen tons of people let me get away because they instantly hook to me and flay instead of waiting for me to use my gap closers first.
No more space