Been lurking for a while but made an account to comment on this. My primary concern is that being able to see what other people think of something might mean you don’t bother to read it yourself; downvotes have the same effect however their broadness of why a post might deserve one means you need to actually at least skim through something to see if it’s unhelpful, false or badly-worded such that someone could misinterpret it. Downvotes help guide you when you aren’t able to judge the quality of a comment on your own, emoji reactions could tell you how to feel about a comment before you even read it. If you dislike a post more than a downvote can represent, you need to comment on it to explain why it’s bad, forcing you to write out your thoughts - perhaps collect some sources - and most likely proof read them before you comment. An emoji reaction, however, is literally one click away and one scroll later you probably won’t even be able to remember it. A more abstract and slightly paranoid worry about it is that this was used by Facebook to track people, and implementing it in Lemmy could open the door to an instance making modifications and using this for the same thing. I know that Lemmy isn’t Facebook, but that’s precisely why I would rather not have these.

However, I do see a use for these in some communities, namely media-based ones where factual discussion is rare and opinionated (e.g. a community where people post drawings, or links to entertaining YouTube videos), for this reason I think it would perhaps be worth considering as an opt-in for some communities. Because comments typically do not contain media, if you do implement in on an opt-in bases it’s probably worthwhile to allow enabling them for posts and comments independently. Some people may also not like them (UI clutter, prejudice, emojiphobia etc) so you should be able to hide them in user settings.

tl;dr this would make you pre-judge comments and posts, and make people react without explaining their thought process in a reply, however could be nice for some communities as an opt-in