Finally tried Carn Dum last night and it's every bit the grinder I'd heard it was. Great challenge and a cool environment. But...
First, let me grumble a bit about the instant death from contact with the water in Carn Dum. Instant death sucks. Even something like the ocean by Forochel, where you take a ton of damage would convey that you aren't supposed to be there and I'd be totally fine with it. But dying because you cut a corner too close while running around fighting is just silly. That's without taking into account the possibility that one of the trolls will knock you from the platforms.
Which brings me to my next point.
I got knocked off of one of the platforms by a troll. Had I wound up in the water and died, I could have handled that - I was sort of expecting it as I fell. Imagine my delight when I landed on solid ground! Unfortunately, I was up against a wall and surrounded by water on all sides. I couldn't even jump across. My only escape was to basically kill myself by jumping in the water and going back to the start. That's dumb. Being killed by a foe is one thing. Leaving an instance open to being stuck alone with no option but to commit suicide isn't exactly the sort of heroic encounter I was looking for.
Well, you've stumbled upon the point of that entire sub-zone. Item one: local water is provably deadly. Item two: from lower-level encounters, trolls are well known for their ability to knock back. Put these together and you have a challenge situation where your fellowship must pay attention to field so they don't end taking a bungee jump. If the area wasn't a platform jumper there wouldn't be trolls there either. It's donkey kong, only barrels are missing.