Tips to Help you Dominate your Opponents
Changing your hero can mute one or possibly two hero choices from the other team. Imagine you are a playing a very strong Widowmaker: Two players switch to Tracer and Reaper to counter your Widowmaker. As soon as you notice this, or die because of it, switch to McCree for instance. In this case he counters both these heroes well.
Using hero switching well can not only negate the enemy picks, but open up the heroes you play best. Starting as say McCree again to deter the use of heroes that counter your Widowmaker giving you freedom to use her. Be aware of counters and use them to your advantage.
Exploit teams who have players that take fights alone or in lesser numbers. When you are able to start picking off players 1 by 1, 2 by 2 you gain a great amount of tempo and can translate this to objective time. Find the players that are pushing too far, or playing with too much adrenaline/emotion and kill them first every time. This is the easiest way to start the "Pick-Train."
4. The only wasted abilities are those are never used or never synchronized.
Abilities have Cooldowns, be aware of your heroes cooldowns and what situations they excel in. Refrain from using the logic, "I do not wish use X ability because X ability may be needed later" If you never use an ability in the time it would be off cooldown you effectively wasted an use of it. However be wary of using abilities you will later; spamming some abilities will be less effective than if precisely timed. Key point here too is when you feel you are going to die never just run; Juke, shoot, spam your abilities. This will help your team win critical fights.
Synchronizing abilities has devastating effect, a well timed ultimate can outright win a game. Know the heroes you are with and what synergies you can exploit. Key examples here is Zarya's Ultimate, which acts as an AoE Vacuum crowd control ability, and Pharahs ultimate, which rains an onslaught of rockets at your foes. If these two ultimates are used in tandem your enemies will have to figure things out back at the respawn area. Here is a working list of strong combinations to use in your games.
5. Coordinate and work with your team. Overwatch is a team game, and much like fighting as a team with using numbers to our advantage, we want to synergize our abilities, pushes, and picks. Think and work as a 6 man team, communicate with any force you wish. I found during the beta the emotes were particularly useful (especially the one letting your team know the status of your ultimate).
Lastly! Eat well, Drink lots of water, and be ready to enjoy the most entertaining shooter of the year!