People will watch a Let's Play when the narrator is engaging, informative, and entertaining. If the latter of the three is fulfilled, sometimes they don't even need the informative bit - it depends whether you're going for a "teach people what the game is about" channel or a "play through and share the experience" channel.
The only LPer I followed for any length of time was Raocow, and it had nothing to do with his skill or the games he played - I watched because he was funny, and every video had something to be entertained by.
He was very casual about everything, and watching his videos was much like sitting in the room with him while he was playing (and being very quiet). It helped that he was non-sequitor and quirky, but that's just his character - it's not like he tries to be.
Point being, just do what you do when you play a game around a bunch of your friends. Since you're doing it with someone, sometimes it's entertaining enough for that sort of channel to just have casual banter as you get through levels.
Or something like
this.
