I won't miss many individual subs, but I will miss the totality of communities.

I will miss that, if I had a problem or question about X, there was almost always a sub for X, and it was usually the best place to get information from people that have some sort of clue.

Example: when it came to weird behaviors from my Samsung Odyssey G7 monitor, /r/Monitors was the only place with clear, focused discussion about it. The Samsung web forums had some people complaining too, but nobody actually sticks around the Samsung forums to have ongoing discussions, so getting a full picture of what people are experiencing was a lot harder. Plus, those kind of forums are always filled with a lot of Yahoo Answers quality of posts, so you have to sift through so much junk to find any usable info.

I've had this experience so many times with Reddit. That's what I'm most afraid of losing.