Just as a comparison, these are the month's top posts of the Rocket League subreddit:

http://i.imgur.com/NI3mPKX.png

And these are the ones from /r/Overwatch:

http://i.imgur.com/GT9Gzoe.png

I was harsh in some cases as I wrote the descriptions to the right of the posts, but my point stands: Too much garbage and low-effort content gets upvoted on the OW sub. Honestly I didn't want to include even more posts because it would've been too many or too big images.

On /r/RocketLeague we get a good mix of funny moments, mechanically demanding highlights of e-sport tourneys/professionals in general or high-ranked players. You also get some useful suggestions that increase the accessibility of the game (QoL changes). Other than that, loads of fan content and guides. Low-effort posts either get removed or don't go too far up. The front page often has highlights of average players (mostly up to 200 upvotes), and it's good that they don't get too many upvotes. The best plays and effortful posts are the most visible. I don't say that there are no garbage posts, sometimes people rant or write "I did it [achieved a ranked goal]" posts, but they don't reach the top usually.

Even though the game is easily accessible for casuals, just as Overwatch, the focus still lays on E-sports and improving at the game.

On the other hand we have /r/Overwatch. We got lootbox gifs, mostly from the last event. This is extremly low-effort: you just open a lootbox and show it to Reddit. The cases in Rocket League at least give away some special flair, you can sell some ingredients for over 100$. Also, you won't see much of such posts since there is /r/RocketLeagueExchange.

You got Fan Content, that's fine. It's something that was made with effort usually. You'll see it on the RL sub as well. Nothing to say about that.

Highlights on /r/Overwatch are often Q of the games you don't really need skill for, often with some extra bit like an emote or something. Torb/Symm afk potgs, ragdolls, team kills. It's always the same bs. Granted, posters try to make them as unique as possible, but in the end it's essentially the same garbage. In Rocket League you also got flicks, redirects, jump resets, trickshots in general. The difference is that these things firstly need the use of techniques (which means you need to be good at the game, not really much of luck or "I pressed one button, upvotes NOW" involved) and secondly you can learn much from it. Sometimes you even learn wrong things if you watch /r/Overwatch highlights. Flanking as McCree can end well if you get picks with your ult, but it takes a lot of time because of his lack of mobility and if you die or don't manage to kill multiple people you just wasted that time. Now people try to get those flank deadeye potgs over and over again which doesn't improve them as a player in particular and also the matches they play in general concerning match quality.

Meanwhile, /r/RocketLeague highlights teach and inspire you to learn advanced techniques to do such highlights as well. That way you usually have to do something, namely getting better at the game, to make a good play and reach the front page.

Humor. I like the humor of the RL sub more, but it's sometimes low-effort content as well, but not that much as the OW sub, because on the RL sub people at least don't make shitty jokes over and over again (MEI = SATAN XDDD I NEED HEALING XDDDDDDD)

You rarely find E-sport content on the main sub since there's /r/CompetitiveOverwatch, also there is /r/OverwatchUniversity for guides and such. The community, that way, is divided into those who want to get better or are interested in tournaments and those who are just there for the memes and the low-quality highlights. This isn't the way to go. Don't allow this shit. "Flood" the main sub with useful content and some day even casuals and meme guys will try to improve the game because they got influenced by the content of the front page. Allowing garbage posts is just worsen the problem.

Suggestions. On the OW sub you see the same SHIT suggestions over and over again. In the end, the effect on having a voice line for eating ultimates as D.Va is just marginal, while non-transparent goal posts in Rocket League are a big drawback. Also, people there often post suggestions with a [Repost] flair as long as it's a repost, because the people there now how to use the search function of reddit, unlike the usual /r/Overwatch users. Suggestions in general are tied to QoL changes in RL, while in the OW sub it's garbage that doesn't have a real effect anyway ("Bacon" voice line for Junkrat as an example). Useful suggestions get buried by this garbage on /r/Overwatch.

Mods of the OW sub need to enforce the rules, either flair reposts as such or remove them, ban low-effort garbage, promote e-sports (they do with "This week in e-sports", but it's not really doing something in my opinion). Suggestions and complaints should be removed if they're not opening room for discussion. Overwatch is a game designed for everyone, so it's actually more of a casual game. But so is Rocket League, yet the community, including their reddit sub, is mostly influenced by content creators (guides, cool fan content, not garbage memes) and good players/professionals. They don't need various subreddits for different streams (competitive, university), because they do it right.

TL;DR: Do something about the sub.