Well I've been spectating quite a few games lately. I usually watch the streams on own3d quite a bit, but that only gives you one player's perspective. The spectate gives you everything.
One thing I noticed. 9 times out of 10 the reason for a team to start losing happens around level 4 to 6. And damn near every time it's a sequence where someone thinks they can just get that kill if they push in harder. They push in, seeing someone low and die trying, then every one else on their team that is near by or trying to help follows in. Then they all die.
That balls deep attitude loses games the vast majority of times. People just don't know when to back off. No one plans an escape or anticipates help from the other team coming over.
What's worse is once that happens they don't change their play style. They do it again and again. But by that point they are now getting so far behind they are screwed. Now the other team can charge in with impunity and tower dive at will. Then it just snowballs further.
All it takes is for one lane to screw that up or the jungler to chase just a bit too far on a gank. Which usually happens from level 4-6. Sometimes it goes until level 8-10, but that is rarer. At least if people wait until level 8-10 before doing the balls deep bullshit, they have a chance to recover. When you do it earlier, you have no chance to recover. The other team snowballs too fast and you fall too far behind.
I'm not aiming this post at anyone. It is just a general observation I've seen many, many, many times now.
The other thing is people don't learn to cut their losses and compound deaths.
When someone is doing something obviously stupid to get themselves killed. Let them. One death is far less severe to the overall outcome of the game. When 2, or even more die on a team at the same time that is a massive advantage gain to the other team. Massive. Gold, exps, time spent free farming, free dragon kills, and possibly freely able to invade another lane and snowball another kill.
Which brings me to another thing, the team with the first person to reach 3 deaths on a single champ is the one most likely to lose the game. There are statistics to back this up, but I can see it happen every game.
Lastly, one final thing that makes a fairly big difference is map awareness. Especially backing up others on your team. The team that pays attention the fastest and gets together in an area first is going to win. Hands down. If you are mid and see your jungler in a duel in the jungle you need to break off farming ASAP and help your jungler. The lose of a little CS is minor compared to the death of your jungler. People don't seem to comprehend that. If your jungler dies, that means the enemy jungler is that much stronger. The enemy jungler can now free farm your jungle for a bit as well as theirs. Your jungler can not gank when dead and is now falling behind. When they fall behind they can't gank or gank well. They can no longer put pressure on the lanes. The enemy jungler can put pressure and has a higher chance to make a more successful gank.
Same thing for the jungler. If a lane is getting ganked and you are on the same side of the jungle, you need to get over there asap. Screw buffs. Screw farm. GET THERE. The junglers that do save their team most of the time. The junglers that don't lose.