Besides game mechanic reasons, why is it that some words hold a can of whoop-ass and others don't? As you know, the Word Walls are memorials and gravestones mostly, with epithets and tributes to an event or soul laid to rest. And some words on those walls stand out to your Skyrim character (I'm assuming they recognize them in some visceral way... having a dragon soul themselves). In some cases, it seems plausible that words like "Force" and "Fire" and "Storm" might carry some weight, but in others, it's puzzling. Why is "Kyne" a shout? Actually looking over all the shouts, "Kyne's Peace" is fairly unique. None of the words are militant or explosive, but they carry magical power.

Could be that Kyne is a god? That might cause it to be a word of power.
Also, since it is basicly magic, it could be that certain words are what causes the magic to happen. So in a debate between dragon 1 and dragon 2, certain words is what causes things to pop out of the box.
For example:

Dragon: I hereby announce that I'm forfeiting my life for this cause. (Where forfeiting, life and cause. Are the words of power)

I'd bet it has to do with the belief behind it, and less about the actual words.

Or a Dragon's understanding of it. Dragons can't use Dragonrend because they don't "understand" it. The idea of mortality is incomprehensible to them. I'm assuming if the concept behind any given word could be fully comprehended by either a Dragon or Thu'um Master it can use it as a Word of Power.

Personally, I was under the impression that any word in fancy dragonspeak had the potential to be a word of power when understood properly, but certain words stick out more readily to different speakers more than others based on personal affinity to certain concepts and whatnot. The words of power we're shown, while they have plenty of facets that could take decades to explore the hard way, tend towards being pretty basic concepts that a man/mer/funny little scaly fellow could easily see the value of weaponizing. Though in-game dragons basically use the same shouts as the player, lorewise they've had ages and ages to ruminate on the finer points of grammar, so by now they're probably weaponizing conjunctions against the math-nerd Hist trees or whatever dragons do when they're not landing on cities and eating adventurers in funny hats. Meanwhile, the Dragonborn isn't much more than an egg and is only just now learning to speak in anything but baby-babble.

So basically, I think that intent, understanding, and creativity of the speaker makes some words powerful and others not. It's been awhile since I've touched Skyrim though, so that could be completely wrong.

What I wonder is why the dragons, when speaking to you in dragonish, don't inadvertently let off shouts. Ah well, they should at least have made the typical rumbling thunder sound whenever Dragons or others spoke to you in the dragon tongue. Just like they did when the Greybeards call you "Dovahkiin".

That's why I was thinking there's a different between dovah language and Words of Power specifically. Same thing goes for how you read walls.. only the words of power stand out. There are many words and names in those texts, that are more "conversational" in natue.