I'm just one guy, but here's what I look for when I watch Let's Plays:
Are you funny/entertaining? Are you and your friends going to be giving commentary about the game or something for additional entertainment value? Most people aren't going to want to just watch some people play video games without something else going on. Good example of this is Shamus Young from this site. He has a Let's Play series on his blog that ranges from hilarious to insightful to vitriolic. It is very entertaining.
Are you doing it for something or is it just for fun? Now it seems like this isn't the case, but still. There are several groups that do game marathons for charity. For example, LoadingReadyRun, also from this site, does the Desert Bus for Hope every year. Bad example for a game, but they do a lot of other stuff along with it. There are other groups that do stuff like that.
In fact, here's a group doing something like that now:
http://www.justin.tv/thespeedgamers?#/w/1177732256
Not the best time for example as I am posting but you can see they have a couple cameras set up, a chat stream and are doing stuff based on the comments from the chat.
A third reason which doesn't apply to this is if I am stuck on a game and want to see someone do it.