Who would classify Path of Exile as grindy or not challenging? Do you just use hte same abilities all the time? All the fights the same? Is wining/losing about some random critical or you misjudging the linear power of the enemy, rather than about some tactic or something legitimately intriguing? Is it just a game of increasing your numbers, but little else?
I'm in another thread the a player in POE who reached max level apparently says the game has no challenge. I'm coming here to see if that's a widespread feeling or just a minority viewpoint. I myself haven't plaeyd PoE.
Grindy? To some degree, more if you insist on playing only with the items you find yourself and not trade with anyone. Then it takes time to find some really good items (Not that you need them, you can access most of the end game content with just good items).
As for the skills, at the endgame you tend to use maybe one skill that hits a single target hard, one that hits a group of monsters, a curse to make monsters weaker and some kind of mobility skill like lightning warp, ground slam or something similar. There are also some minor considerations like a skill linked with increased item rarity that you use to finish rare mobs and bosses to get better items.
Yes, there are builds that only use one skill and a curse, like Arc builds or flameblast fire prolif builds, but there are also other builds that utilise more skills, to for example generate different charges.
Winning/losing is more about knowing what a mob can do, if it has any nasty modifiers like damage reflect or powerfull criticals. The tactics aspect comes more into play when fighting bosses that have special attacks and different abilities that you should either try to evade or deal with in another way. I would not say that you just need to increase number and it will be a smooth sailing throughout the whole game, merciless difficulty will very quickly show you where you are lacking both in your build and the knowledge.
And I would definitly not say that the game has no challange. Sure, if you are level 100, optimised build with great items then only a couple of things like reflect (If you didnt build a countermeasure) or corrupted blood can kill you, but very few players ever get to that point, and we still see people on levels 90+ dying so the game is plenty challenging.
I would agree that for an experienced aRPG player, normal difficulty will be quite easy, and maybe only the act Bosses will be of any problem, but that is more due to the player being good and making sensible choices as I've seen shitload of new players struggling even on normal.
But for an experienced player, the endgame maps, Atzire runs and ultimately Uber Atzire are where the real challenge is.