Even if you have all of those things, sometimes it's just a hell of a lot of fun to watch a good Let's Play. Some of the better ones I remember for years, and I have a feeling that one which showed a lot of promise but was discontinued far too soon will bother me for years to come as well.
Agreed - I have to mention necroscope86 on YouTube. By far my favorite LP'er. I think he had a snafu with his account that deleted a bunch of videos, but he had some very funny X-COM play throughs. It was more about his commentary and British accent that made it worth watching than the actual games themselves.
Some of my favorite LPs have come from LtMkilla (Dead Space and the ill-fated Bioshock LPs come to mind). His humor probably isn't for everybody (whose is?), but I love his videos.
If we're highlighting great LPs, I'll have to throw in a recommendation for ChipCheezum's LPs of the Metal Gear Solid games. The humour is spot on, and the games themselves are covered in exhaustive detail. His LPs of Mega Man Legends and the Uncharted series are great too.
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.
