All the mycology subs, but I'm thinking of rebooting them here.
All the synthesizer subs.
All the subs with eerie places like r/AbandonedPorn, r/RustyRails etc.
I'm going to miss browsing r/all for a good laugh and interesting stuff.
I'm going to miss stumbling upon random subreddits with topics and interests I didn't even know existed like r/Jarrarium, r/DesirePath or r/ThatBathroomMazeDream