I am now on nightmare difficulty with my level 48 barbarian and I start getting bored. I can illustrate one of the problems in pictures:
level 44, but still looks the same at 48
The only difference of my barbarian, apart from the weapon she carries, is that I used dye to change some colors. The Armor is exactly the same for the last 13 levels.
I picked up tons of loot. White, blue, yellow, doesn't matter. Not a single piece of armor I picked up for the last 10 hours I played was as good as what I am already wearing. And by that I mean useful
for my character. I find ton's of rare weapons and armor with bonus for intelligence. Too bad I don't play a wizard. The secondary stats on the pieces are usually useless for wizards, like +strength or the generally pointless "increase gold pickup by 2 yards". Hence it doesn't sell in the auction house.
I could buy some armor in the auction house you say? Spendid idea! But wait, there is one important problem: most people don't have a buyout-option and those who have want a stupid amount of gold for a slightly better
pair of boots than I already wear. Most auctions I find either run out in the middle of the night, where I can't just sit there and refresh to make sure no night-owl takes away my presumed bargain, or they run out in 2 days. In 2 days I'll be of much higher level already, hence the slightly better piece of armor which will cost me a fortune will be out of date.
ARPG like Diablo rely on steadily growing numbers. Increasing levels, increasing damage, increasing armor, increasing amounts of enemies. If at some point this process of gearing up stops, it shows how hollow the game actually is. At nightmare it's still not difficult. I die quite often, but that's not a result of difficulty. It's because the game has plenty of cheap deaths. Since everything is random, it can happen (and to me it happens quite often) that one rather small room contains three packs of champions and minibosses, with insane combinations of powers.
For example one room with a pack which could fear my character and did so constantly. The second pack had the ability 'waller', which build a wall around me which prevented me from getting anywhere, while being feared, hence having no control over my character. The third pack had the ability 'mortar', which does what it says on the can and rained a continuous volley of fireballs on me. I have more than 10.000 hitpoints at that point and have a ability that reduces magical damage by 20%. That didn't help me from being stuck and watch as my health-globe dissipated within a second.
Often there is no tactic I can use to prevent me dying. What I did in some cases was charging into the middle of the pack, fire all my abilities, use 'earthquake' (which does massive damage and continues to do so even after I'm dead) and hope that at least one of them dies. Rinse, repeat.