I have beaten both games once. Is there any other reason or motivation for second playthrough? Just for the record I enjoyed my first ride, mainly because of the writing.

I like to play any game I like more than once, so I may not be the best person to weigh in on this. However, just based on the elements of the first game, I could see there being a reason to play the game again on a harder difficulty since some manuscript pages can only be found on (I think) the hardest difficulty. If you don't want the extra difficulty and you are like me and don't try to do every single thing on a first playthrough, you wouldn't even have to bump the difficulty up to experience more of the game. I don't think I've ever found all of the manuscript pages available on the Normal difficulty. Also, there are the collectible thermoses, though whether or not they are worth playing the game all over again for isn't for me to say.

I've been through the game three times shortly after I bought it. Yes, it is very linear with no real alternative paths (going left or right is not a real choice!) so you're not going to see anything new the next time around. The game is quite intense and in your first playthrough you were probably riding some emotional highs and being too concentrated on survival that maybe you missed out on some great lines or other subtle details. Did you know for example that outside the farm there are three graves from which the buried skeletons will emerge and be flung at you in poltergeist attacks? I only noticed on my third playthrough because I was too rushed by enemies the first two playthroughs.

Anyways, I'm busy with my 4th consecutive playthrough of GTA V. Yeah, I'm that kind of gamer.

The first game had a few manuscript pages that could only be found on Nightmare difficulty (which I just beat). Some add more details to the story and some are just funny ("Alan reads a page").

It's kind of like an interactive movie. Like a movie, the only value in replaying is to enjoy particular scenes again or pick up subtle details you may have missed.

Personally, I'm playing other games through that I haven't beaten and then I'll replay Alan Wake again.

It's a shame that y'can't play Nightmare difficulty in your first playthrough. I think that would have improved my impression of the game considerably.

Because, let's be honest, there's not a whole lot of replay value apart from that difficulty. Where's the suspense if we're already aware of the twists?

Exactly. That's why I always prefer the hardest difficulty for genre like this. Beside the collectibles and manuscripts, there's nothing else to motivate me to complete my second playthrough.

There's not even a cheat to unlock it. Unlockable difficulties work in character-action games (but even then a hard mode is usually available straight away), but in story driven action games?

It's odd, clearly they intended replay value with an unlockable difficulty and extra manuscript pages, but the idea just didn't mesh with the rest of the game.

For that reason I slightly prefer American Nightmare, even though it is more action-oriented. At least the hardest difficulty is available straight away, and the horde or arena mode is fun for killing some time since the combat is quite excellent.

I was hoping upon completion there will be extra stages like Resident Evil 4. But perhaps that was bit too much.

Actually, there is: AlanWake.exe -developermenu unlocks a menu where you can select this.