This post is going to come across as rant-y, because it does tilt me how many people have so many wrong ideas about overwatch and yet are often strongly convicted of them (mostly by lower skilled players but I see it in higher ranked games as well).
 
1. The 3 people on the payload meme is often over-prioritized.
First of all, the difference in speed between having 1 person on the payload, vs having 2 or 3 isn't very large (ages ago it used to be, but since they buffed the speeds it's not). This is unlike capturing a control point (the difference in capture speed is noticeably much faster with 2 or 3 people on compared to 1, also having more than 3 on capture points doesn't make it cap faster). I'm sure if you've cared to pay attention to this, you've noticed how it doesn't seem to move much faster when you jump on it. That's because it doesn't. According to this guy's post, these are the stats:
 
Capturing a Point:
 
1 person = 100.0% speed
 
2 people = 150.0% speed (when compared to 1 person)
 
3 people = 200.0% speed (when compared to 1 person)
 
Escorting Payload:
 
1 person = 100.0% speed
 
2 people = 116.5% speed (when compared to 1 person)
 
3 people = 133.0% speed (when compared to 1 person)
 
Each additional person is only increases the speed by 16.5%.
 
Having 1 person on the payload is pretty important, but if the 2nd or 3rd person on the payload could be doing something more important like killing stragglers, peeling for allies/supporting someone else killing stragglers, helping a teamfight, or the most commonly overlooked yet incredibly important, repositioning to setup for the next fight, that's usually better than marginally increasing the payload speed. You ever notice how much pro teams prioritize getting into a strong, spread out position with only the tanks on the payload, while they are preparing for a potential next fight as the payload nears the objective? I don't see this nearly enough in comp mode. As a general rule, if you're a squishy get the fuck off the payload, that's how you lose the next fight and they fully hold. You're a 76 or mccree pushing close to the 2nd point of Route 66? Get on the left high ground platform.
 
2. related to one, but over-prioritizing the objective, rather than winning the next potential teamfight.
You're a zenyatta or ana attacking Point B of Temple of Anubis, you've won the first initial fight but the enemy team likely or even might have a real chance at defending. Get off the point and back out to a safe and annoying spot for the enemy team to deal with.
 
Or on defense, on the opposite side of that scenario where your team is wiped but you've time to get back on, if you're one of the ones who can first jump onto the point, just fucking give them extra ticks and capture percentage, what's important is you being able to fully hold, and that means waiting until the last moment where you'll have more people who can go with you together. Stop just rushing the point and dying immediately, that's the strat when you've basically no chance to hold and can only stall for a bit of time. Also try to always think about what hero you may want to swap to as early as possible (you can choose your hero while dead, don't wait till after you've respawned). In scenarios where you can maybe hold, avoid picking stall heroes but focus on getting kills, even trading 1 for 1 is a big win as their spawns are much further than yours.
 
Apparently at lower levels, there's a meme about not getting on the point (especially for hanzo/widow/genji players)? Yes in most cases, those heroes (and more) shouldn't get on the point, instead focus on kills/surviving/winning teamfights; they're what matters for capturing objectives! This should go without saying, and yet this meme is pervasive. I don't get it honestly.
 
3. Lack of patience/discipline when it comes to peeking/poking, and over-valuing the benefit you get from peeking compared to the risks and the tradeoffs involved.
This is mostly for when you're attacking. Please if you're down a player or more and your teams are disengaged, just wait and hide until he comes back, unless you're confident you have a good reason not to. Many times, while you're gaining ult charge by poking them, you're also feeding them ult, but more importantly you're usually way more at risk at getting killed. Also especially if you already have your ult, and they're playing conservatively/safely and unlikely to die, why are you poking at all? This stuff should be obvious yet I still see it too much.
 
4. Not paying attention to allies enough/focusing too much on enemies
The most detrimental but mostly only done by lower skilled players is moving/diving in too early or without being aware of, or looking back at your allies' positioning. Often due to impatience at waiting a few more seconds for the rest of your team to arrive. But the more common mistake is during a teamfight, especially a chaotic one, tunnel visioning on enemies and not paying attention to where or what your allies are doing. If you're being aggressive on them/diving on backline, dont just go for whoever you can, go for whoever your allies are on. Pay attention to your teammates, especially your backline/vulnerable heroes and peel for them. Sometimes that's hard so it's very important to call when you need help as early as possible, and try to say your positioning. If you start paying more attention to what your allies are doing rather than just enemies, you should notice a difference in your effectiveness.
 
5. Being too concerned about over-extending, and not seizing an advantage when you have one.  
This is mostly only done in lower skilled games.
 
Generally when you have a 2+ player number advantage, you're going to win, so attack aggressively. If you won the fight and it's a 4v2 or a 3v1 on a KotH map and you can chase them as they run, yes go chase them if their allies died recently, you're not in danger and getting more staggers does help. Of course pay attention to how long it's taking and how close you are to their spawn though. Especially if you have a lucio. And if it's KotH in this meta, get a lucio.
 
6. Minor yet common mistakes:
Stop trying to live when the fight's over. Escape if you can, or just die ASAP or you're wasting your own clock. Also Hog stop Taking a Breather you just feed more ult charge and stall your respawn even more. If you're a DVA who can calldown mech but the fight's over, don't bother. And if the fight's over, just die to their ult if you can, they don't gain ult charge while ulting, especially if you're a tank as you feed a lot of ult meter, so jump off maps if you can. If your spawn is way further than theirs and you've your ult, don't even try to get a kill or shoot them, it does nothing for you except maybe feed their healers ult meter.
 
Also yes, it is worth it to stagger that last remaining Dva pilot (assuming they're not stalling you from capping/pushing payload), so restrain yourself and dont kill her as long as possible. You can damage her a bit, try to keep her from moving/pull her back with hooks, boop her back to you, etc. You'll also want to comm this over voice as well.
 
Finally a last point of general advice: Whenever you're in doubt of what you should be doing or how to play something, just think about what's annoying for the enemy team or what annoys you the most when you play against that hero or when you have to attack/defend on X map. Then do that.