Sequals already planned

I suspect they're still in the planning stages, but given the plot they've revealed already, it sounds like it could be a Silent Hill kind of thing. Same principles in each game but new environments, opponents and maybe a new protagonist.
If the main charcter finally solves his problem in the game, maybe the next one can be 'Ellen Wake.'

I'm curious though, if they're making such a huge game, does that mean that there will be side-missions, challenges etc unrelated to the main quest? It's going to seem like an awful empty map otherwise, unless the main storyline is really that big.

No, I didn't think there'd be any taxi missions in it.
Little side missions maybe with mysteries to solve, like Oblivion crossed with Max Payne are the way they're going to go, I'll bet.

True, although this sounds like it's going to be fairly tightly plotted, so any updates (barring patches) would probably have to be little novellas in their own right to be satisfying. I'm guessing that the sub missions are going to be something like Dead Rising, like you said, all sorts of little places to explore, maybe things or people to find that the locals point you towards.
I wonder if the locals are going to realise there's something weird going on or if it's all going to turn out to be some weird alternate reality (like Silent Hill) or all in Wake's mind?

Well 'Remedy' don't make failures, they have only made 3 games currently and they have all been runaway hits, i think they are very confident in Alan Wake whats not to like about it.

Free Roam
Action
Horror
Thriller
Natural Disaster

All in one game and then to top it off, graphics better than any game on the Xbox 360 to date, 'Remedy' are on to another winner i think, and its good they are planning ahead.

5th of January as far as I've heard.. the other one was January 30th.

Its because they now have the power to create photo realistic lighting with little to no taxing on the GPU or the CPU, so they will use it like anything now, but Alan Wake is the first game to ever use such advanced volumetric lighting which is an incredible effect.

Yeah i am still hopeful of January it is also a Windows Vista title and currently that operating system only has Halo 2 to show off on it, Vista needs a game like Alan Wake in January to show that its worth the upgrade.