I have been playing a bunch of Path of Exile lately and I have been impressed with the way the endgame works. In particular I am impressed with how well POE compares to Diablo 3 when it comes to endgame progression and the difficulty curve. When playing D3 I just smashed through all of Normal, Nightmare, and Hell without stopping and then suddenly got brutalized when I arrived in Act 1 of Inferno. With a combination of playing better, figuring out better skill combinations, and some gear tweaks I managed to get Act 1 down pretty fast and then hit the brick wall of Act 2. Not only was I dying a dozen times to every elite group but it was also abundantly clear that it would take a truly monumental amount of farming to change that. The prospect of farming Act 1 for a hundred hours to allow me to be able to slowly and painfully make progress in Act 2 was extremely unpleasant. The difficulty curve was just a wall rather than being a smooth increase throughout.
POE seems completely unlike that. As the level of zones goes up the monsters get consistently more difficult; although there are definitely prime levelling places I definitely find that I get a little more gear and a few more points and then I can move on to the next area comfortably. The progression into the map endgame seems smooth as I was able to tackle the first map I found at the listed level. I died some but a lot of that was just cockiness and inexperience and once I took the enemies seriously and regeared a bit it was quite doable.
The key here is that I consistently get the sense that I am getting better. I get a new piece of gear and then I can farm the Docks instead of the Fel Shrine. I get another piece of gear and I can farm the Cathedral instead. Another piece yet and I can start effectively farming my first maps. The trouble with the Act 2 wall in D3 was that even if I got several upgrades I would still be doing exactly the same thing as before - clear out the whole second half of Act 1 again. There was no feeling of progression, no sense of being able to do new things. Having a really good difficulty scale that gives that constant sense of progression is key to making the game fun.
A major factor in making that happen is the inevitability of damage. In D3 it was quite possible to avoid nearly all damage and building a character that would be one shotted by everything was the only sensible choice. In POE that isn't reasonable because you are going to take damage and you must be able to survive it to accomplish anything. That allows the difficulty curve to be much easier to construct because if the monsters do 10% more damage that *matters*. When all monsters do infinite damage it becomes extremely difficult to actually ramp up the challenge of encounters in a granular fashion.
Ahead of time I hadn't realized just how important a feature this was but it definiltely is - there needs to be a lot of damage flying around that is nigh impossible to avoid and which the character has to be able to survive to make sure that being tough enough is a constant struggle. A smooth progression curve is key to a fun experience in a RPG and achieving that is so much easier if you can do so just by ramping up the damage the players take by 20% and be sure that doing so really matters.