3. Warding -
I can't stress this one enough! Far to many times people will die from 5 PEOPLE GANKS!!! After i someone calls the mia on everyone! You might think buying wards for other people is silly or just a waste, you might even think buying wards for yourself is silly. WELL sir, you shouldn't be playing ranked games. Think of it this way
Sight Ward - 75g
Vision Ward - 125g
If you ward your lane, let's just say you throw 2 wards down no matter what lane (usually top/jungle/bot will usually ward 1 place), that's 150g which can add up pretty quickly. However, to counter this, let's assume both teams are playing great and just a small gold advantage will make a huge difference. Well if you ward (150g) and you prevent just one gank in a 3 minute time span, you are effectively ahead 400+ gold (in most cases). You might say "just play careful DURRRRRR", playing careful almost always means losing CS, even if it's just a few, it will add up.
If you know where that enemy ward is, and stomp it, Vision Wards are effectively 100g. Now that they don't have a ward it opens up ganks, which means 300g (usually, unless they are feeders, in that case you should be winning anyway) + assist gold. Vision Wards are great for preventing and getting ganks!
4. Communication -
This comes in various forms, the 3 most common are; Trolling, Raging, Ping Spamming!!!!! Those being negative are still better than no communication at all!
It's always good to - the game with a "HF & GL" usually followed by some pre-game joking and fun poking from both sides. Then I usually tell my team what's up, I tell them what lanes i'm going be focusing, if I'm going to gank at level 2, either for kill or just pressure. It's IMPORTANT for the jungler to let people know to COMMIT, so many kills have been lost because people didn't commit, and I will take blame for not communicating that!
This doesn't just go for junglers though. It's important for everyone, especially when countering the enemy jungler as a team. So many games I have 1v1ed the enemy jungler at their second blue, no one came. A simple ping from the jungler and action from the mid player would have atleast stopped me, now they are doing a kill there will be a blue for me and my mid :D!