It's all about personality for me. Well, skill at the game also factors in, I'll get to that. Just like radio, Twitch streams hinge upon the charisma of the person/people participating in them. If you've got a bunch of fucking Opies doing a stream, telling each other to shut up all the time and playing in silence, yeah, it''l get old real quick and you'll wonder why anyone even does this. But if you get a great streamer who can either carry interesting conversations with teammates/enemies/whoever, or even hold the viewer's attention by themselves, it's a treat to watch. Their level of skill in a game is important too. I personally prefer two extremes in this regard: people who are very good at games and can push them to their limits, or consistently produce exciting moments, or people so hilariously bad at the game that goofy, memorable shit happens regularly.