A "pit of death", for the sake of this discussion, is a pit that a player or an npc/monster dies if it falls down. Usually there is nothing at the bottom of the pit, other than death. These pits appear in many games, but typically don't exist in vanilla skrim (as far as I know)

This can be good because a player can do stuff like faus ro dah a monster into the pit.

On the flip side, its possible for the player to be knocked into the pit, maybe a good thing?

There are also some issues with the player's followers being knocked into a pit of death. In this case the player's follower has died in one blow. Also, if there is no alternative way into the pit, the items on the follower are lost. A modder can script it so that the follower is only injured and then teleported to a location that is outside of the pit, but this may break the experience's verisimilitude. It's also possible to drop important items into a pit like this, but vanilla has an area that you can drop items in and never get back to (Thalmor fort).

What are your opinions on putting something like this in a mod?

I don't like pits of death. It's the cheap man's difficulty. Spike traps? Sure, but not pits of death.

Now, if we're fighting on a mountain top or someplace high, I do expect that I can fall over and die

There is at least one place where you die when you fall, or simply venture, into in vanilla Skyrim; killed by spikes. I don't see why you wouldn't add some to a mod... It's good for... plausability.

I love bottomless pits and spike traps. I cut my teeth on Mega Man and Crash Bandicoot in my youth. Skyrim is no platformer, but still, I love pits.