Overwatch, like all online multiplayer games, has some general rules of etiquette that should be understood - you don't quit a match mid-game, you don't try to fuck with your own team (more generally through teamkills, or - in Overwatch - using stuff like Mei's icewall to impede your own team), etc. But Overwatch has some rules that are specific just to it by itself - rules about kindness and respectfulness that we should all try to keep in mind when playing.
1. Always vote
It seems kinda crazy that this even needs to be made explicit, but here we are - especially when a team loses, more and more I see people exiting the match before they vote, or staying in and just not voting (presumably out of spite that their team didn't win, or just failing to recognize HOW AMAZING I WAS AS REINHARDT). Just like in real life, you should vote. It's your videogame civic duty.
2. Always vote for your supports
End of game voting will typically present you with a few options of players who did exceptionally well in the match - whether they spent a lot of time on the objective, did a ton of damage, or - most importantly - offered a lot of healing/support.
Your support's job is not showy, and it's certainly not filled with the kind of praise that comes with getting the usual quintuple kills that garner Play of the Game. But it's necessary, and they do it humbly. People don't play Lucio, Mercy, and Zenyatta for personal glory - they do it for the good of the team. So give them your vote, even if the dude playing McCree did get 53% kill participation.
3. Always vote for someone on your own team
Listen - you may be DAMN impressed by another team's player. It might even be a situation where they personally destroyed you in a way that SHOULD cause a lot of spite and frustration, but ends up resulting in you respecting them and their playstyle even more. Regardless, you don't vote for them. Give them the "gg", say "nice play," whatever.
Overwatch is a team game, and you offer your vote to your team, even if they didn't play their best. Loyalty means something.
4. If none of this applies (no one on your team was an option, there were no healers, etc.), just vote for whoever
Voting and winners literally have no impact on the game, so it actually doesn't matter WHO you vote for - it's just a nice ego boost, and you might as well give it to your teammates when you can. When you can't, there's absolutely no harm in giving the other side your vote.
5. Don't cover your teammates sprays with your own in the pre-game portion
C'mon, don't be a dick - I just sprayed my super-sweet Symmetra sprite-style spray over the pre-game window, don't cover it up with your shitty Reaper spray. Go spray somewhere else.
6. If you team does not have a healer, you be the healer
This cannot be said enough - OVERWATCH IS A TEAM GAME. Maybe you really, really want to play as Soldier 76 - but your team lacks a healer. The thing is, your team REALLY is going to need a healer and you can't order anyone else to be one. So swallow your pride and support your team by playing as a healer.
7. Don't bitch about how the other team was being 'unfair' (particularly Bastion)
There's - roughly-speaking - about a 50/50 shot that at the end of the match, Bastion's gonna get Play of the Game. That's just the nature of a robot-turret who can wipe out Reinhardt's shield in 4 seconds flat - he's gonna get a lot of kills in a row when given the opportunity. And while Bastion can be a frustrating hero to go up against (especially when they're hiding behind a corner and blow you away before you EVEN REALIZE WHAT'S HAPPENING - not that I'm bitter or anything), but the fact is that he's not perfect, there are plenty of counters to him, and he might just force you to re-consider your strategy. After every death you are given the opportunity to change your hero - that's your opportunity to take care of Bastion.
But don't complain, because complaints are just like assholes: they are about to be blown away by fucking Bastion in his goddamn turret mode.
8. Be prepared to play as whatever hero your team really needs at that moment
There are characters you probably don't like playing, but end up being necessary for your team to succeed. And since we were just talking about him, let's use Bastion as an example.
The Bastion hate is real, and it's not hard to see why - he's not a particularly difficult character to play, yet he racks up a huge amount of kills regularly and really isn't that fun to play as. Unfortunately, he's a necessary evil on pretty much every Defense Escort mission - and there are (rare) occasions where no one wants to be the boring, guaranteed Eliminations-winning bird-lovin' robot.
In those cases, you gotta be like Batman at the end of Dark Knight - not the hero your team deserves, but the one they need right now. They may hate you, scorn you, complain when you get POTG (even though they shouldn't), but when you get that sweet "VICTORY" end-screen, you'll know that you did what was right.
9. Don't tell someone they need to switch heroes because you both picked the same one
There are two key things to remember when playing Overwatch:
1. It's a game, so chill out.
2. It's a TEAM game, and you're a PART of that team. Note that you're not the LEADER (despite how high level you are), so you're not in a position to be giving out orders of who someone should pick.
Do you think your team needs a Reinhardt? Don't tell someone to switch to Reinhardt - YOU switch to Reinhardt.
10. Communicate - whether by voice chat or text, offer strategies or just friendly suggestions
The best way to play Overwatch is as a team - heroes are meant to complement one another and work together to overtake the opponents, but too often it's just every man for themselves hurtling themselves towards the objective with no thought as to the best way to group up. Mercys running off without a tank to follow, Soldier 76s heading into a firefight without a Reinhardt to provide cover, and Torbjorns trying to set up their turret without anyone else around to provide a distraction.
Talk to each other, come up with strategies! The more you communicate, the better chance everyone will have at winning (or, at the very least, not being quite AS frustrated when you get totally owned by a Bastion for the umpteenth time). But going along with this, communication is a two-way street. Don't bark out orders - demanding people switch to specific heroes or roles, telling people they're playing awful, or making any proclamations that don't invite discussion and cooperation. As intense as an Overwatch match can be, it's just a game.
The Unwritten Rule (which is now being written, oops): BE KIND
Overwatch is a game.
Repeat it to yourself: OVERWATCH IS A GAME.
Your teammates are not always going to be great - sometimes they'll be downright terrible, or purposefully troll-y, or just confused. And while it can be frustrating, remember: OVERWATCH IS A GAME.
Not everyone has 8 free hours in the day to practice and git gud with every character. And sometimes even YOU will have a bad match or two - and when you do, you'll want people to not rake you over the coals for it. You'll want them to be cool about it, because after all - OVERWATCH IS JUST A GAME.
Be a team player, vote for your teammates, and don't complain. And, as in all games, BE NICE TO EACH OTHER.