Runes - Rune Pages - Masteries - Summoner Spells - Champions
Any player having more than the other in those categories has a clear advantage if they use it.
Next would be Experience.
Because it takes players so long to get their hands on everything it will create and separate the player skill base for quite some time.
Players not having adequate resources of the game is generally why they end up in Bronze or other tiers slightly better such as Silver - Gold- Platinum. They get lv 30 with limited resources and knowledge and make a mistake to que up in rank and end up in Bronze/etc. Players need to be taught better in a competitive e-sport. The problem, Riot isn't teaching. Its up to players to teach players and to make do with what they got.
League doesn't even have a training mode that gives you access to all champions you own to test out all their skills and item combinations with levels and how to react to certain scenarios such as flashing or evading over walls/terrain or performing skill shots on fast moving targets unlike how most competitive fighting video games do. Riot wants to be a competitive game but doesn't want to work on helping their players get better faster. After all its the mass amount of casual players that bring in the cash. Because Riot has no answer for toxicity. They would not be able to prevent casual players from getting flamed and alienated from the competitive players. All Riot knows how to do is treat cancer but not cure it.
There are several ways to eliminate this games toxicity. Casual and Competitive Que. Normal and Rank.
The issue is Normal has became infected with Competitive players. Rank is a disease that has spread to normals due to rewards.
Rewards make players want to win. To get IP to buy new champions and the Runes they so need and want to play.
Rewards cause even more toxicity in normals than they should.
Because of Rank, the meta was born. No longer can you go against the meta and not get the evil eyes.
Chat is a way to also spread this cancer. After all they say give your cold to someone else to get rid of yours.
Look at that I don't even have a PHD!.