I think it really just takes a bit of practice to getting into how you would want to commentate your videos and get into the way of things. No harm giving it a go and see how it goes. Might be easier to do post narration, as in record the footage then narrate. At least for me I find myself with more things to say once I've recorded than recording live but I find it more fun recording live & having fun is one of the main parts of it I thin, obviously having others have fun while watching is good too mind you. I really want to play Digimon World again, been years and is still one of the few PS1 games I still have, even though apparently the PAL version can't be completed so I guess I can't finish it, would be good to see a Let's Play of it though.
Not really got much to add myself but a lot of the ideas mentioned already sound good, I quite like the sound of a Youtube channel, even though as mentioned really depends on how people would be dedicated to it. The badges and extra help threads sound like good ideas as do the challenges.
I understand the not knowing what to comment on for LP's as in general I mostly just comment on LP's by friends, even though I do watch a good amount of them. Then again I am a fairly quiet person when it comes to that sorta stuff, I guess others might be or there just isn't anything in particular to comment on.