For me, it was the toxic circlejerk (admins/mods as well) low quality content getting upvoted while high quality submissions not getting much attention/buried and obvious privacy issues.
I still use reddit as it has content lemmy doesn't.
But yeah I think there is a lot of bots/shills that manipulate the conversation on reddit that doesn't really happen on lemmy. This will happen on lemmy too if it gets big enough.
Federation is also cool. If lemmy does end up full of bots/shills then someone could make a new instance with some new rules that help to reduce the number of bots (e.g. answer some questions about a post to comment or something).