Cinematic fights tend to take place in interesting locales. Movies love to use cliffs for this purpose because they really want the villain to die in the end but having the hero stand over the villain and deliver a final fatal blow to a fallen opponent doesn't fit well with many folks. They want a hero who is an expert in violence and who smashes faces with ease but who won't actually finish the job and a cliff is the standard answer - the villain can, at the last second, do something aggressive and stupid that causes them to fall to their death sparing the hero the necessity of murder. For extra bonus points the cliff can be replaced with a boiling pit of lava or a pit full of snakes / crocodiles / scorpions or some other lethal danger.
Obviously fantasy roleplaying games often end up the same way. The tops of castle walls, perilous mountain passes, the heart of an erupting volcano, or a rickety bridge over a pit of spikes are standard locations for fights with terrifying monsters and mad wizards. The tricky thing about these sorts of fights is that there are so many ways for them to end really rapidly. DnD Next, DnD 4th, and Heroes By Trade all make use of effects that push and pull people around the battlefield and this means that instant death is but a single push away. It is troubling because normally pushing somebody a few squares away is a pretty minor thing until it becomes an instant death attack when they fall into lava and crispify.
DnD 4th has a rule that if you are going to be pushed off of something you can just fall down and avoid the push entirely. This is a pretty good way to avoid the issue but it does seem unfortunate that pushes can be sometimes even less useful when next to a cliff than they normally would be! It also is completely bizarre that you can't avoid a push normally by falling down but if a cliff is nearby suddenly that option becomes available. DnD Next seems to not have any rules of this sort and any fights next to deadly dangers are often going to result in instant kills on round one using a trivial ability. HBT is in that same boat currently and that concerns me - I had a situation in one playtest where I had to remove an enemy's Power that would have knocked the players off of a cliff and made the fight they were in pretty near unwinnable.
I don't like the idea that cliffs suddenly change the rules of the game. Ideally the rules play the same way regardless of the terrain but pushing people around isn't quite so deadly. Knocking people down instead of pushing them is a reasonable solution but then you have to resolve what happens on the second push - is that one now lethal, does it do nothing, or something else? I am kind of leaning towards letting people avoid pushes by taking a ton of damage, between two to five points of damage per square of the push. This would make pushes absolutely fantastic in cliff type situations but wouldn't end up killing tough people right away. It does make some thematic sense too - instead of trying to mitigate the blow you just stand there and take it on the chin. This would still mean that mooks who get pushed off of cliffs just die anyway (which is fine!) but serious opponents can survive the situation while still very much fearing those push attacks. Seems like a solid compromise to me.