I miss forums. When they were in their heyday I was an active participant in anywhere from a couple to half a dozen, shifting with whatever happened to be my hobby at the time. And local forums based around hobbies like music and photography were a great way to meet people in person because you already had something in common to start things off.

It was also a place to find really in depth information on a topic. I remember doing research for my multi-day hikes and outdoor travels by browsing the threads in the stormfront survival subforum (note: I do not condone what they represent, but lots of them were paranoid and preparing for "the coming race war" and they just had good prepping and survival info).

To me Reddit and HN have filled the void left by the decline of forums, but it's not the same. Perhaps the thing I miss the most is the ability to have avatars and custom signatures and titles to give your online persona a little bit of personality and flair.