Movies with twists.

Watching movies with twists again...hmmmm.
Yesterday morning before work, a bulletin on MySpace had made me think about the movie Saw, so I decided to watch it again. I'd bought it ages ago with its follow up and watched them together as soon as I could, but yesterday I decided to watch the first one again. No sooner had I put it on though than I remembered the majority of the film.
A brief resume without spoilers for those who won't watch it, or haven't seen it yet.
'Mad' person has imprisoned 2 people in a room and demands that one kills the other before a set time or his family will be killed. There are no particularly horrific moments - though it's an 18, and there is no real tension in the film either. It's predominantly a dialogue between two people, discovering how they might be connected, why they are there and them trying to figure out if they can escape.
It's a good film the first time, I did enjoy it, but after watching it again I realise I don't need to watch it a third time.
This is a conundrum, for films like Se7en and Silence of the Lambs stand up to several viewings. Either this means that Saw is a particularly brilliant film that ingrains itself, or it's actually just a film I spent £3.99 on and should have returned.
In order to test my theories further, I have started to watch Saw 2 this morning, but it's as I suspected. I know what happens too well to really enjoy it. Again, the first time I liked it a lot though. I remember the 'extras' on how 2 was made were really interesting though.

So, what makes a film re-watchable?
Comedy? Well, things like Some Like It Hot are always as funny as the first time. And I think I know most of Wayne's World off by heart. This Is Spinal Tap is another I can watch again and again.
Tension? Apart from those mentioned already, heck I'll include Spiderman - because I can.
Tragedy? The Great Escape, Papillon, Night Of The Living Dead, Assault on Precinct 13. All re-enjoyable.
Drama? Well, Spidey again, but it's the closest I can think of to 'drama' that I watch.
Horror? I will watch really awful films for the amateurishness of the special effects and the stupidity of the storylines, but they will go into the comedy section in all honesty. O instead there are films like The Omen and Boys From Brazil that are horror I suppose.

Is it something else, or a combination of some or all of the above?

Oh, there's no answer to this as everyone thinks differently and has different opinions, tastes and dislikes. So it's just outpourings of thought as the detective tries to find his son and the location of the house where the miscreants are trying to escape...