How do you guys feel about encounters designed to be tpk unless you already know what's coming and are actively setting up your party to take advantage of where you'll know enemies will be/know the mechanics of the game really well?
They work, if done right. In fact, "boss" fights tend to be like that. If you can just walk into a fight without knowing anything about it, and still manage to figure things out on the fly and win after one or a couple tries with whatever tools you have available at the moment you initially walked into it, then it's not that hard. And if the game has only fights like that, it's not fun. Not all bosses have to be like that, but at least the "elite bosses" should be able to wipe you out unless you prepare specifically for it. Such encounters are still fun as long as after knowing "oh so they can do that", you can come up with something to counter the enemies, and the satisfaction comes from having come up with your strategy and executing it right. You agree that it's a puzzle kind of fight, and the satisfaction comes from figuring out the key and solving the problem. Personally I find this exciting. Thinking about the fight, trying to come up with my own ways of solving it - a plan, then gathering the necessary tools (which may require you to go somewhere first to buy stuff before coming back later), then putting it all together, seeing if it actually works, adjusting the plan, until it finally works. It's the same process when trying to master anything.
BG/BG2 have plenty of fights like that. Pretty much all SCS lich fights, dragon fights, hidden boss fights. Sometimes end-of-chapter fights. Most if not all of the more challenging fights that involve mages. I play with mods, but it's probably about the same thing as a casual player playing vanilla.
But it's not fun if the game doesn't provide you with solid tools to
reliably solve the problem. Enemy tricks that simply can't be countered in any way and all you can do is reloading until you get a bunch of good rolls.