It's a lot of little things. The feel and sound of the tiles- which Mahjong Soul and Yakuza capture surprisingly well. Rules that are reasonably straightforward on their face but full of fascinating exceptions. A fair bit of randomness, but tools to manage that. Really good pacing. A scoring system that just feels rife with the marks left by the people who played it before.

I'm doing a bad job of explaining. I don't really know. I just enjoy it.