This is the same state of many subreddits on Reddit. Moderation should be limited to spam and blatant attacks on users. Let the voting system handle all the rest.
Despite scrapping the office "defaults" list, there is still clearly a rotation that get set up as default and they are by and large garbage.
I mod a good-sized sub all by myself, and I'm fairly militant about what gets through. The guidelines are clear and automod picks up a lot of the 'defaults' stuff -- but every so often something slips through and it gets so much karma.
A while ago I figured I'd just allow memes and stuff, assuming that this was what the community wanted, since these occasional posts were doing so well. I got mod mail asking if I was sick, noticed a bunch of comments saying 'this sub has gone to shit!' etc etc.
... so I went back to my militant ways, but about once every few weeks I let a meme or repost slip through if its decent quality.
Memes and all that have their place, but subs need to be heavily curated by the mods, or, like you said, they all become the same.
The occasional memey or off-topic post is a lot of fun and key to having a community with its own culture instead of an interest group with the personality of a dictionary entry. The problem is that people see that was fun and naturally try to do more of it, but it's only fun in the first place because of the contrast, and the off-topic stuff is usually easier to produce so it can easily drown out the on-topic posts, killing the forum.
The "delete most memes" policy is weird, but works well where I've seen it.
Does that bother you? Why? If "so many" people like something, why should you or any handful of mods prevent them from seeing it?
What keeps you from deleting stuff that only you don't like, regardless of whether or not it "belongs" on the sub?
Do we just want an echo chamber?