So it's not a bug, it's a feature.
Yeah, its turning out pretty much exactly how I imagined as well. No risks. No dread. Lots of tactical illusionism. Heroic feats attempted not because the situation demands you take the risk, but just because you can and because well they really aren't risks. Even players not being able to make sound judgements because the game universe bears so little relationship to the real one. The worst sort of cornball action movie as the standard of dramatic tension. It's all there. Nerfworld. Everything bounces. Everything is now the 'I can jump off the 60' cliff because it can't really hurt me' problem.
My suspicion is that the game is something like Diablo II now. The only thing that kills you is boredom. That is to say, you get extremely overconfident, stop being even moderately cautious, or stop paying at all attention because the last X fights have been repetitive and that's what kills you.
Now, as far as the actual encounter goes, that's really cool. I'm all for runaway carts and splating darkmantles on your shield. But I'm not sure that I'm happy with the idea of the player/character anything but terrified when doing so, and I don't see how this encounter couldn't be ran in 3E with 6th level PC's or so, setting DC's and CR's accordingly. Essentially, this is a long series of forced CR 1-4 encounters occuring in rapid succession. It's not a new feature. You could have run it in 1E using the old mechanics of Dex checks and save vs. paralyzation.
Good encounter design doesn't show off the new system as much as they think it does. It just shows that they are using sophisticated encounter design. Bully for them.
As for splatting darkmantles, well, the trick would be to have them strike perfectly perpendicular to the shield. If they did that, then the impact would be about equivalent to a 20' fall. Since the darkmantle only has 6 hp, its quite reasonable that the impact could kill them. But I'd be very skeptical of your ability to do anything but a glancing blow to a darkmantle while swooping along at 30mph. But even if we rule only 1d6 damage, thats still potentially enough to kill one, and the flavor of the desription is worth the slight exaggeration IMO.