I think it varies in terms of what a given viewer is looking for, but generally having a voice I can listen to for long stretches, some amount of humor, and competence at the game you're playing. You also need to be able to commentate on it in an interesting manner and do interesting things - nobody wants to watch a Let's Play when they don't understand what's going on, and nobody's going to sit through an hour of you strip-mining some mountain in Minecraft.
But people like different degrees of all that stuff - I generally find YogsCast to be too silly, but thousands of subscribers obviously disagree with me, and I'm sure there are Starcraft or LoL watchers who hate all the contextual strategic commentary that I find absolutely vital in those videos. Similarly, different people and different formats like differing amounts of audience participation - I usually watch stuff on YouTube or Twitch weeks after it was recorded, so Q&Aing with your chatroom means I'm missing half the conversation, but for guys watching it live that's pretty awesome.
