I'm rather surprised that people would say they couldn't rate it based on not knowing the game. You don't have to know the game to rate how well it was recorded, how the gamer reacted (was their commentary too over the top, too quiet, too loud, etc), was their audio equipment okay, was their lighting for their facecam good or bad, etc.
If I asked someone to review one of my let's plays, I would expect them to pay attention to stuff like camera angle, background lighting, sound quality of my audio... you know, stuff actually pertaining to the video itself.
If I asked someone to review one of my let's plays, it would never occur to me to think that they'd review the game or how I play it or if I editing stuff out or not. That's stuff that has no bearings on a review of a video at all. If the reviewer thinks they have to review the game itself or how I play it or what type of edit cuts I make, I'd seriously question their ability to make a quality review of any video, gameplay, vlog, or otherwise, because that's not what reviewers do.
Video reviewers are NOT trying to review the game, and if they don't know that, then they really shouldn't be offering to review other people's videos at all. They are trying to review the video, the gamer, his/her set up, and give the content creator feedback on how they can improve the quality of their video. It has nothing to do with the game or the let's play style or the editing style. Each game has specific style of editing and cuts and playing, that someone not familiar with the game would not understand well, so those sorts of things should never be considered at all when reviewing a video. Same goes for vlogs and beauty how tos - each has a style of editing and presentation that is their own, and should not be considered as part of what is being reviewed, when someone reviews the video.
How you review a pure gameplay channel, as in, one that just shows certain actions, footage or such with no commentary or little editing.
Long form RPGs tend to have no editing and also are usually 1 to 3 hour long episodes, but they tend to have a lot of commentary and reactions going on from the gamer. You'd want to be reviewing their equipment and set up. Is their camera angel "off", is their lighting casting shadows that makes it hard to see their face, is the sound of the game so loud you can't hear what the gamer is saying, is the game audio so quiet you can't hear the fights happening, that sort of stuff.
Don't worry about commenting on the gameplay itself or commenting on their commentary.
If they have no commentary but have a facecam, perhaps they are shy or distracted and unaware they are going quiet for extended times. Point that out. Say something like: "I got confused as to what was going on. I don't know the game and I wish you had been talking about what was happening with the story so that I could have followed along easier"... gamers often forget they know the game's storyline and plot but the viewer may be new to the game and not know what's happening.
Pointing out what's going on, reminding the viewer "So in the last quest ___ captured ___ and now we are looking for ___ that's why I'm going in here...." that sort of commentary helps viewers follow the storyline of the game as the gamer plays it.... if you as a reviewer were clueless as to what was going on and thought the gamer had no commentary even though they had a facecam, then go ahead and tell them you were confused and couldn't follow the story and would like to see them do more explaining the story in their commentary.
That's valuable feedback.
ah... but you see, knowing sound quality of their videos is EXACTLY the type of info gamers want feedback on, especially considering how many tracks of audio they have to piece together for each video. It can be difficult to judge getting the game sound low enough so you can hear the commentary and the commentary sound loud enough without drowning out the game sound. Feedback on sound quality is something gamers look for and want.