PvP. The second they start bashing away on monsters, I'm out. This isn't because I hate PvE, but because it's predictable - I know what's going to happen. In PvP you have no idea what's around the corner.
They must also have some sense of viewer awareness. Just pause for a moment and read chat to let them know you're paying attention, otherwise we may as well just go watch youtube videos.
I mainly watch people I know. If not I prefer personality over content. Content has to be something that I am at least somewhat interested in though.
I dont watch many. I watch a streamer for Arma3/GTAV(roleplay). Been watching this fella for like 6 years now. Because its a funny guy. And well, the things he streams usually end up quite funny aswell.
For ESO I only really watch Alcast If I watch any ESO streams. Because he is a nice guy, Used to play together alot. Apart from that, I am not that interested in ESO streams. For me ESO as stream content is not that interesting.
I prefer pve streams so I can see what other players do in the content I enjoy and how I compare to them plus look for handy stuff I did not think of myself. What makes me follow and regularly watch a streamer is how they interact with the audience and their personality. I'f a person is boring to watch I will not watch the streamer unless I have a certain question for him/her or something. The eso streamers I watch the most would be tabatta, alcast and karma.