Successful Let's Play videos all have one thing in common: the person playing has verbal diarrhea. You need to let your mouth run for 30 minutes at a time, talking about nothing and everything, focusing on the important aspects of the game and filling the pauses with funny.
I'm not going to tell you to stop making those videos, but I'd ask you to watch some LP videos from "professionals" and take notes. Watch some of TotalBiscuit's "WTF is..."
videos and Jesse Cox's "OMFGCata"
Let's Play videos.
Now, if you're going to do co-op Let's Play videos, watch some of these
. Two dudes rolling around in a game, unable to do anything right, but it's still very entertaining.