I haven't watched many LPs, but generally I watch ones of games that I've already played. I don't really care to watch someone else play through it in regular fashion, unless it's something like a puzzle game with multiple ways of solving it. Being witty is a must, since it's essentially an animated lecture.
if I want to seriously watch the game for the story, then no I don't want a peep out of the recorder. If they want to make side comments its more appropriate to make it one of those notes, now if I want to watch, for example amnesia and I want a good laugh, I'll sure as heck watch someone scream and make jokes, funny as hell but not when I want to read story on some game I prob will never buy or I just want to see a different ending to a game with multiple endings
The only reason I would watch a "Let's Play!" is for tips, tricks and hidden stuff to help you along.
Like showing the hidden upgrade capsules in the Megaman X games.
This. Or to watch the walk through if I'm stuck at some point. I don't need your jokes or funny observations at every other second. I'd prefer you leave it to a comment or two once in a while, where you explain some error/bug you may encounter at a specific part, or a secret, or something useful. When I'm browsing for something to watch, as soon as I hear any commentary, I go back and search for a different video instead.