There's a definite pain/pleasure threshold. I don't mind being knocked around and bounced about as long as I'm enjoying the ride. Vekoma Corkscrews are a good example of this. The never really "hurt" me, they're just a bit of a bang, but it's never more than a bit of a slap or something, but the rides are quite good fun under it. I come off feeling a bit battered, but I've had a little fun too. If I came off with a headache or bruising, then the pain outweighs the fun.
Wooden coasters I expect a bit of a bashing and I love the bumping around a ride like Megafobia or Big Dipper (Blackpool) gives because it's harmless and the underlying ride is so much fun it doesn't matter. I agree with madhjsp and Gavin that Cyclone just fits into this, but not at the back...
Grand National is the same - in some seats/trains it's fine and the bashing is part of the fun, but in others it's horrendous. The pain is overwhelming and my last run on both The Nash and Cyclone I would have happily leapt from the train rather than suffer it any more. They really did push me beyond acceptable limits.
Poseidon at Europa Park actually left me with a substantial bruise on my leg from one rough bit (and the rest of the ride was **** ), so that's a big no-no of a ride for me (though I would ride it again anyway if everyone else was
). Baco and Kanonen I agree with CD on. With Baco it was the shaking and the fact it did bollocks all. It's essentially just a figure 8 with a slow inversion while all the time you're having your brains beaten about your skull. Rita shakes me about too, but at least it mixes airtime, and fun high speed corners between the beating. Baco offered nothing after the launch but feeling like a rat in a blender.
Kanonen was similar. Far too much jolting and unpleasant hang-time and nothing good about the coaster at all to make it worth the pain. Dull launch, piss poor top-hat, short layout of pain. Not huge pain, but more pain than any element of fun.
I guess essentially, if I can still come off a ride smiling in spite of the pain, then it's fine. If I come off just with pain, it's failed somewhere.
Nemesis back left always chews my ear off badly for instance, but I don't care about a boxed ear when the rest of the ride is so excellent.