I watched the first video, and it was decent enough for a first attempt. Since you asked for criticism, I shall try to do so, at least in areas that aren't simply a matter of practice. Firstly, you talked over the cutscenes. That's a massive no-no right there. Never talk over the cutscenes, that's one of the worst things you can do as a Let's Player. This also goes for any story related in-game dialogue. Remember, a Let's Play is about showing the game first and the player second.
Next, the audio. I found that, especially during the firefights, I could barely hear, much less understand, the commentary. You may need to play with the volumes of the game and commentary during editing to ensure the volume ratio between the two is always good. For example, during a firefight, you may need to turn down the game so the commentary is clearer, but then during a cutscene or dialogue, it may be wise to turn the volume up again.
As for the commentary itself, as others have said, it's largely a matter of practice. That said, it may help, at least at first, for the commentary to be semi-scripted. Or to put it less pretentiously, think about and decide what you're going to talk about before recording, maybe even jot down some notes. It should be a lot easier if you don't have to improv the whole thing.
Finally, try to show of everything you possibly can and play at least at an above average level. That means any secrets and easter eggs present. Don't be afraid to research any beforehand and do some practice runs, and as Chit said, edit out any failures and derp moments. Oh, and if something like a cutscene fails to play, don't ignore it. Find another way to record it and edit it into the episode. Hell, dig it out of the games files and splice it into the episode if that's what it takes.
Hope that helps, friend.
