Saw an interesting question posed elsewhere which was 'Do you think watching a Youtube playthrough entitle you to determine if the game is bad?'
Maybe I can pitch in here, since as a cowardly person, most horror games I have ONLY experienced by watching them on YouTube. And Last of Us + Heavy Rain because I don't have the right console for them
I actually have a list of the ones I've seen (holy shit that is a lot of hours):
Silent Hill 1, 2 & 3
Clock Tower 3
Cryostasis
Call of Cthulhu
Rule of Rose
F.E.A.R.
Comdemned: Criminal Origins
Siren
Thief 3 enter the cradle level
Heavy Rain (well, only one playthrough)
Amnesia
Dead Space
Dead Space 2
System Shock 2
The Last of Us
Penumbra Overture: Episode 1
Penumbra: Black Plague
Based on all this 'experience' (lol) my answer would be NO, you can't determine if a game is good or bad if you watch a playthrough. You can determine if the story is bad, perhaps, but in the end it's about the overall experience. I don't remember a lot of stuff from these games anymore because I only watched them. If people talk about the games, I don't feel like I can pitch in because I have a very shallow knowledge of them. And for me, the whole point in watching the horror games on YouTube is that the effect of watching is not nearly as intense as playing them. If it was, I wouldn't be even able to watch them. The time I remember feeling most anxious watching something is not even listed here (Scarecrow in Arkham Asylum, when Batman is hiding from his gaze). Heee...
Also a lot of games that are great to play are not great to watch. o-o Watching a game with a lot of combat scenes (some of the games above have those, too) is a very dull experience, and a lot of the time I was multitasking instead of intensely engaging with the video... It takes an extremely skilled and experienced player to do even combat in a way that is entertaining. Oh and of course, none of the playthroughs I watched had commentary.
I have to admit tho, with a game like Rule of Rose it was possible to tell that the controls were clunky simply from watching it, and I think some things like that can be picked up from watching. But watching is so not the same thing as playing >_<