Let's Plays have never been interesting to me, though. Either they feel like a waste of time because I'd rather be playing, or they feel like a waste of time because the game doesn't interest me enough to want to play it.

For Factorio, I like that there's a progression. There's an end goal to the game (build a rocket and get off the planet). Mechanisms have a purpose (efficiency and multiplication of player effort).

In vanilla Minecraft, I built things like hidden bunkers that would open up holes in the ground when you push a button, just because it was cool, and I imagine I'd have a similar goal if given more tools; maybe I'd build a TNT factory that auto-loads a cannon and blows up big parts of the map. In Factorio, I'll build a huge train network as part of a pipelined production process, with each train programmed for specific routes, conditions under which it leaves specific stations, etc. The goal doesn't feel the same, and it's nice that the game itself doesn't have the same feel as Minecraft.

I've also only played vanilla Factorio, but it has official support for mods, so we'll see how flexible those end up being. I don't know much about the API that's available to developers, so I can't speculate how much they'll eventually be able to change the game.