I've collected some knowledge from around the web about positioning in Overwatch and wanted to compile it into an easy-to-reason-about framework for both you and I to use. So without further delay, let's dive into this five-point guide for improving your gameplay by obtaining better positioning!
Safety is the most important part of positioning. If you die, you can't provide value to your team.
Never die from a D.Va bomb again. Make it harder for your enemies to deal damage, give your healers some time to relax, and lengthen your lifespan by being able to take cover when expecting incoming fire.
Being close to your healers means that you can stay alive longer which is obviously a good thing. Also, by being close to your supports, you can protect them if an enemy attempts to flank them. If your healers are preoccupied or dead, having a health pack nearby can be a lifesaver.
With high ground, it becomes easier to hit head-shots on opponents, which the game rewards you for with more hero damage dealt based on a multiplier.
High ground also enables you to take cover much more quickly than on low ground, since all you have to do to hide from enemy fire is take a few steps away from the ledge.
Another advantage of high ground is that a lot of heroes will have a hard time getting up to you as a result of their lack of vertical mobility.
Make sure you keep the high ground as long as you possibly can without endangering yourself or leaving teammates vulnerable, as it's generally much harder to get high ground than it is to lose it.
If you're in trouble, are you close enough to your team for them to assist you in time? Could you be helping your team focus-fire essential targets? Are members of your team overextended and could you remind them to mind their positioning? This sort of team awareness will help you stay alive longer and win more team fights as a result.
A lot of heroes have limits in terms of range in both their primary attack and other abilities. For example, even though Zenyatta's attack orbs do not have damage drop, his orbs of harmony and discord have a limited range to be applied to targets and you must maintain line of sight of your target for that ability to persist.
For projectile heroes, being too far from your targets will contribute to worse aim as it's hard to predict the where a player will be at the end of a projectile's long flight.
Also, there are roles where you will need to adjust your positioning to be able to provide the most value. For flankers, you should be in the back line of the enemy to do your job, for supports you should be behind your team to take less damage and have sights over the whole fight, and the list goes on per hero and role.
Chokes are places where the enemy team is likely to funnel through to get to their objective. These places are usually small doors or gateways that lead to the next stage of a map. Since you can predict that enemies will be traveling through these points, it's essential to exploit that and focus your damage there when you have the chance. Making sure that you're positioned in a way to take advantage of these low-risk, high-reward situations will help you secure more kills and be of more value to your team.
By using this mental framework throughout your matches, you should be able to improve your positioning immensely and live to provide more value for your team in key moments. I wish you the best of luck on the competitive ladder!