Atmos Duality said:
Good delivery and organization.
Organization and editing is what separates a good Lets Play from "Dude talking about stuff while playing".
This, so much. Go and film 3-4 hours of content if you want to make a 1 hour video. Being liberal with cutting out not-great parts means that you'll have a better video in the end.
OP, if you're looking to make some Let's Plays and you're looking for hints, check out the Roosterteeth Let's Plays and the recent Full Plays. In the Let's Plays, there are ~6 people trying to accomplish a given task (build a tower of pimps in Minecraft, win a match in GTA). The focus of the video switches from one person to another so you're not watching pointless filler, and cuts entire sections if nothing worth noting is happening.
The Full Plays, on the other hand, are different; more like what most people think about when they hear "Let's Play", in that it's one person going through a game with minimal editing. They're good, but not great; you never realise how much of a game is filler until you watch someone trying to be entertaining while backtracking for the 5th time. Don't be afraid to cut the boring sections and/or re-dub audio to make a coherent story that skips over the non-essential part and delivers the best experience.