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.