This is going to be a short blog post just because I have started college again and my focus is on other things now.
This is another big issue I see in matches.... Do you really know all the items in the game? If not and you plan on being "good" you should learn them because you might be the reason why you completely lost the match for the team. Tell me how many times you've heard this "adc" before the match has started then you notice that your adc has amazing farm but they aren't actually doing much damage? This is due to them not knowing which items are best for the situation. Let's say the enemy team (talking about LoL) has no armor, what item should your adc be getting? I'm not even going to answer that because you should be thinking of items that melt people who have little to no armor. Now take that item and combine it with another item that ignores armor so with those two items calcucate how much extra damage your adc will do if he just looked at the enemies' items. Everything is about what items you have to change the tide of battle if you notice your supports are dying a lot it is likely because they have no items but if they have items yet got stuff they didn't need then they need to learn which items fit the battle. Say in LoL the opponent's adc is a very high speed attacker. The support should be grabbing items that slow down the attack speed or gives aura effects to help the team.
Knowing items is something that every player should learn about since you never know when you might have maxed out items and have to think on the fly to sell an item for a better one. This happened to me today in bot matches I had to buy a new item for almost everytime I was killed in battle. I was able to beat the bots but it gave me a ton of trouble because of the constant change of knowing what items would help me right then and there rather than saying I got max items and don't need anything else. Saying that I don't need any other items would have caused me to not gain more damage for less speed while my team prepared for mass aoe attack speed slow and movement speed slow. Knowing this I was able to easily pick more damage over being able to attack fast but no damage and worry about dying from not enough damage fast enough that would have caused me to die from spells.
Knowing the items to me is just as important as knowing the champions or heroes. Knowing that Axe needs a blink dagger to be very effective over getting a vanguard first will mean Axe is more of a threat than not being able to blink and call. Many players I watch randomly will get vanguard first and the team will be behind in initiation for another 5-10 minutes unlike the Enigma that has been able to gank with his blink dagger for the last 5-10 minutes gaining easy kills for the carry. Getting kills should be the biggest objective and helping the team earlier is better than thinking about yourself and what items will help you live longer. You should always put the team before yourself... Even though you may be the carry if you know your initiator is missing his blink dagger at 20 minutes you should let him farm the gold to finish it so it gives you a better team fight. If you are thinking about yourself you will put yourself in a position where you are not a team player anymore and blame people for things you actually were in control of.