3rd most important skill. Itemization.
There are many examples I see every day of sub par optimization:
- Continuing to build dmg when you're capable of going tank when behind
- Not buying vision or upgrading trinkets
- Not realizing that buying too many damage items leads to diminishing returns has dimishing returns
The biggest issue however I see in most games lies with adcs. Brace yourselves.
Lemme repeat. In the majority of games, you're not going to see enough standard teamfights to make use of 
How many games do you get both sides with a tank top, jg and sup? Or even 2 out of 3?
How many times do you face off as against 
top 
mid or any combination of teams that have at least one dedicated diver and are pointed straight at you.
The purpose of is so during extended team fights you get a force multiplier by causing aoe dmg, i.e. damage to multiple heroes with a single auto attack.
TLDR: Believe it or not. In Soloq, the correct attack speed item to buy 75 % of the time is 
Just because you went 
or 
does not mean you must rush