I'd be very interested in such a thread. Not DMed for years ('84/'85) and back then we just followed the books and modules. Now with the claimed flexibility of 5e I see I may have more work this time around. I've been trying to follow this thread, so I guess my first port of call is to really bone up on the DM guide and the encounter guidelines bearing in mind that there *may* be an issue of being underpowered depending on the team.
Are there any guides that people can recommend for optimising PCs (again back in the day it was just "Whoah! That spell/armour/weapon sounds awesome. i'm getting that." with no real regard to optimisation) and also responding to optimised PCs from a DM's POV?
I'm very much of the mindset of "teach a man to fish." To that end, here is some very basic but useful advice that I think is critical to dealing with issues like the ones you mention:
Learn the difference between Challenge and Difficulty. They are not the same thing in a game design context.
Challenge is just a situation you can win or lose via the application of skill.
Difficulty is how hard or easy it is to win.
With that in mind, now understand that for a player's decisions to be meaningful, a Challenge must allow for the players to impact Difficulty. By their choices, they can make a Challenge easier or harder to win. This is desirable because we want players to have an actual impact on the game.
So when you see complaints that D&D 5e is "too easy," the issue (generally speaking) is that the players are making good choices both in their character builds and/or their actions during play relative to the guidelines in the DMG. Optional rules like feats and UA classes, subclasses, or other features beyond the core rules may further push the Difficulty lower. I would also recommend familiarizing yourself with what each of the Difficulty categories (Easy, Medium, Hard, Deadly) actually mean so you can set your expectations accordingly. They are defined in the DMG.
Finally, if you've got players making solid builds and choices and you want to make sure they can't trivialize Challenges all the time, don't forget that you are the DM and you have infinite dragons to throw at the party. If you want more Difficult encounters, then all you need to do is add or modify monsters, utilize interesting terrain, and implement alternative goals accordingly. There are plenty of tools out there to help you do that.
Some people clearly think that this effort should be done for us. I don't even know where to begin with that since it has never been the case with D&D of any edition. D&D is a game
system from which DMs are tasked to build a specific
game. It's something we have to do for our individual groups.