Players creating stuff or in similar ways demonstrating their interest and enthusiasm for the game. I love that stuff. It just makes my day.
Also feedback, good or bad.
But in either case, just engaging above and beyond "shows up on time every week," you know?
I've been blessed by this level of engagement both in the past and present. Player diaries and session journals (a specialty of's). Players collecting funny quotes from the game. Art or other visual artifacts based on the game, like 's fake live-tweets from our first session of Masks, or great character drawings for Dungeon World.
Heck, I love how my players are nigh-constantly posting relevant image memes in the Roll20 chat while we play, or using the drawing tool to put labels and relevant graffiti on the map. It's just great to see the players really engaged.