I agree with @daboross on his posts, trimming the channels down to the amount viable for discord would not be pleasant. I keep #general muted and rarely poke in because of the sheer number of messages all the time. I'm in a few discord servers, but rarely bother trying to participate due to way too much traffic. I don't have time to scroll through every channel to find out if anything is relevent when those channels have activity all day. In slack, the channels being spread out makes it much easier. I can see at a glance if I need to check in on something, for example, if #screepsplus or #screepsplus-support has unread, I will check them first to make sure theres no issues, others like #botarena or #swc I may ignore depending on if I'm busy or not. With discord this becomes much more annoying since so many conversations would end up merged.
I for one, would be much less active if we did move to discord, as others have mentioned they do, I work as a programmer, so have slack open anyway, during slow periods I'll check slack and chime in. I'm also afraid that if we did goto discord, it would end up fragmenting portions of the community, as alliances would be forced to create their own discord/slack/whatever server.
I also agree with @Knightshade here,
We're considering only two options. Seriously. We're taking a considerable hit to organization, group unity, etc. by moving regardless. The last thing I want is six months after a discord move to start hearing people talk about how ... is so much better and we need to move to that instead.
if we are considering moving, we should also consider and evaluate other platforms too, rather than just debating slack vs discord. (I know there are several slack clones out there, don't recall their names though)