You know, I may have to go look at some of these so-called "quality" Let's Plays, because I have been pointed in the direction of the LPs on Youtube by friends/random people who must hate me, and found them to be torterous to sit through. So I was very surprised when Yahtzee admitted to being addicted to the concept. Personally, I hate watching other people play games unless I've already beaten them and/or am there to coach them through certain parts. I hate it even more if they're chatting over the going's on in the game, because most of the time it takes me out of the gameplay moment that I'm trying to get lost in.
I remember G4Tech TV used to have this half-hour show that would cycle through gameplay footage. I really enjoyed it for two major reasons: 1) It let me stare at gameplay in certain games I was curious about or missed altogether, and 2) There wasn't a game reviewer giving me their opinion of the game, I was just watching it and digesting it for myself. The LPs I have watched have done very little to make me interested in whatever they're playing, and somewhat hateful of the voiceover accompanying the gameplay because of the disturbance to my viewing experience.
That is an interesting idea, though, to use an LP for some after-the-fact commentary on a game you've already played. I wonder if there are any good ones for Twilight Princess, which I just finished last month.