I'm by no means a pro-player, currently rank 68, but I've seen enough mistakes from lower level players to share a few tips with you guys. I've played a friend's account from rank 37 to 48 without any losses (about 15 games), disproving his theory of "unwinnable elo hell'.
So here are a few very simple advice you want to apply if you're looking to climb, at any level.
In attack, your main goal is to kill as many as you can.
This is one thing I've seen lower ranks consistently fail to understand. Since you resuscitate quicker and can group more easily on attack, committing to killing 3 people will be in your advantage even if your whole team dies. So you NEED to "all-in" sometimes, I can't stress that enough. People that think you need to stay under the arch of Anubis with your shield up until your DPSs kill enough people are utterly wrong. As the Reinhardt, you HAVE to commit and pick a way for your team to follow you. Doing this below rank 50 will consistently win you the attack.
In defense, your main goal is to avoid dying.
On the other hand, in defense your priority is to avoid dying. By the same logic, if you die even after killing more people, you're at a disadvantage because they'll be on the point quicker. So don't hesitate to BACK as much as you can, even abandoning the point for a few seconds if it means you'll regroup, heal, and slam them right after. This is why the pro teams defend on the high ground on Anubis' point A - whatever path the attack rushes through, they can back off. If they're on the edge of taking point A and you're alone, GET OUT. Dying will allow them to rush to point B with all ultimates and 5v6 to victory. I've lost countless games on Volskaya and Anubis because of people trying to 1v6.
The meta is 2/2/2 - 2 tanks, 2 supports, 2 dps.
By simply playing the right meta, you'll drastically improve your odds of winning. That's why you NEED to be comfortable with at least 1 tank and 1 support role, even if you main DPS (which is my case). Played properly, Zarya and Roadhog will make more difference within the meta than any DPS outside the meta. A good Lucio or Zenyatta ult will also outperform any double kills you would have made with an assassin hero. If you wanna climb, swallow your pride and prove you can carry with something else than your main hero. Remember that it is better to let a lower skilled player play his favorite hero but being as close as you can be to the meta than trying to show him you can get more kills, effectively losing the game.
By the way, Zarya feeds on damage. If a room is filled with junkrat and symettra's turrets & traps, its a free 100 stacks on Zarya's damage boost. You can then just steamroll everyone. Also, if you're a main support, learn at least one 'self-carry role'. Reaper is probably the best example of resistant, extremely dangerous foe you can play if your team doesn't need you as a support.
Communicating before the match starts always helps
At all levels when I'm attacking point maps I usually say a simple 'Let's all go right, if there's a junkrat/symettra covering back and go left'. Even if there are 1 or 2 trolls in the team, in the worst case scenario that will still be 4 people going the same way and likely surprising a random, unprepared dude with his pants down. If your team understands that you'll win 3 consecutive 6v2s even if their team plays better, you got this.
As a flanker, your absolute main goal is to kill the symettra TP and the supports
I've seen a million Genji players just trying to kill whoever they were facing and forgetting about the TP. The TP being up WILL lose you the game even if you kill a dozen people. Cool, you killed 3 people but forgot about the Mercy/Symettra TP. Now they're all there again. Cool, you didn't kill the Zenyatta before your Zarya's ult. Now none of them dies because he ults.
This is typically a case of 'wtf you guys are doing? I killed half their team and we lost the fight!'. Well yes, you killed people but not the right people. So REPICK AND LET DADDY HANDLE THIS.
Look up your stats and be realistic
People are extremely biased when it comes to losing. A friend of mine told me that he was 'always carrying with McCree' but could never win a game. By looking up his stats on one of the websites (there are many, just google them), we realized he had under 10 gold medals for over 30 games played, and very low accuracy and headshot figures. He understood that he was, in fact, the problem in these games.
The bastion dilemma
"They had a Bastion, but our Pharah never killed him once so we lost". Alright, so this is one the situations where you'll have to forget the meta. The meta is cool IF you can win. So pick widowmaker/a genji/a McCree, and kill their bastion until he's disgusted and repicks. Then take back your Tank/support and win the game.
Please, drop Hanzo
I'll wrap up the debate immediately. I'm NOT saying you're a bad Hanzo. I can even tell you you're a good Hanzo if you want. BUT, the skill required to make a difference playing Hanzo versus any other picks is TOO high to justify taking him. This also applies to Widowmaker mains. You may be the best Hanzo in the world, you will STILL not make as much difference as if you pick another champion. So please understand this. I'm not asking you not to play Hanzo ever, quick play is there for you. I'm not saying you're a bad Hanzo. I'm saying if you pick anything else, we'll have a much better chance at winning.
Calculate your ultimates
This advice is probably not relevant before you hit rank 50 to 55, but at lower levels you'll most likely take the habit of consistently getting multikills when playing Reaper -McCree - Pharah. Well at some point, it doesn't happen anymore, because they'll be prepared and expecting you. I almost always hear the reaper teleporting above us. As McCree/Roadhog/Ana, I'll immediately cancel his ult the second he jumps, because I kept my flashbang/hook/dart specifically for him. So be observant and wait for those that can interrupt you to waste their cooldowns before going in.
Ping your ults
ANYONE, please take the habit of pinging your ult status once in a while. The Ana will know when she can boost you as Genji, the Zarya will know when she can ult for Pharah to rain justice on them, etc.
Hope this will help a few of you guys, Gl and Hf!