Moin,
I did not play Amnesia yet, as I'm still waiting for a boxed version (Thomas, maybe you could have a look at my questions
here). Still, I have an opinion on game's length...
I was totally disappointed when I finished BP in 9 hours. Like some others already mentioned, not because I thought the game was too short for 20$. It could have cost 1$ or 100$ -- I just
did not want it to end, I liked the atmosphere so much that I wanted it to continue.
And I think this is the central issue for every game developer: Some people want games they can finish in their scarce spare time within 5-10 hours.
Other people love a game and want to spend 10-100 hours "living" in it... just like me. You will never satisfy both of them with current game mechanics.
In Penumbra, I would have loved some really hard puzzles, taking hours to solve alone. Surely you know Scratches... I just loved to be in that house, running around to find any clue on how to proceed. Other people hated that like hell...
To come to my point: I think the only solution for your predicament would be an option at the start of the game, letting the player select whether he wants to have a "casual" game or the full pain.
As a very simple example: On difficilty "casual", a key to a door lies next to him on a table. On "hard", the player just gets a hint were the key might be, and he has to strive around or solve a puzzle to get it (a poor example but I hope you know what I mean ;-))
By doing so, you would satisfy both user groups with a reasonable amount of additional implementation effort.
Regards
Ozz