I understand the 4e model, but the rat 'fink' DM in me sees those movement forcing effects as 'save or die' without the save.
Whoops, you've been knocked into the spiked pit.
Whoops, you've been knocked off the bridge.
Whoops, you've been knocked off the ziggurat.
Whoops, you've been knocked off the tower.
Whoops, you've been knocked down the stairs.
Whoops, you've been knocked into the acid.
Whoops, you've been knocked into the whirlpool.
Whoops, you've been knocked over the waterfall.
Whoops, you've been knocked into the boiling tar.
Whoops, you've been knocked into the superheated geyser pool.
Whoops, you've been knocked into the piranna infested river.
Whoops, you've been knocked off the boat.
Whoops, you've been knocked off the sky ship.
Whoops, you've been knocked off the cliff.
Whoops, you've been knocked into the lava.
Even, whoops, you've been knocked prone into the scorpion swarm.
If these are fairly typical locations in my campaign (and they are), then I think you can see why I think players deserve a save vs. these sort of movement conditions. In fact, as I play it now, players generally get two saves to avoid being moved even after being hit. The first to stand firm and the second to grab on to something before falling off. That way I can have fights on the rope bridge, in the canoe, on the flowstone above the icy cold pool, in the geyser filled chamber, on the scaffolding where the temple is being constructed, on the floating logs in the saw pond, on the narrow stairs to the hermitage and all the other dangerous locations my combats tend to occur in. People still end up falling off things, but its not an automatic thing for either the PC's or the monsters.