I dunno, as I recall it was less the difficulty of the end boss and more the horrible balancing of the different classes.
First time I reached Titan I couldn't beat him, no matter what. It was impossible.
Second time (with a different class) I mopped the floor with him - at least he was relatively doable.
Third time I couldn't even put a dent in him (I think it was a summoning based class).
Some of the minor bosses were also insanely difficult depending on your build, even in basic difficulty.
I wish there was another game like the original Diablo. It was fun in single player, had enough content to keep you entertained but not so much that it was impossible to get stuff without trading with other players.
I'm referring to, for example, gathering sets - in D2 and D3, you have to grind an ungodly amount of hours for a serious chance to complete ANY set. Lower level sets are completely useless this way, because by the time you grinded enough to get all parts, you'll have outleveled them with half a dozen different characters...
Another thing I loved in D1 and not so much in games like Titan Quest (basically all Diablo-likes after D1) is the world itself.
D1 was still closest to it's roguelike roots, with just the little village and otherwise a dozen (or whatever it was) dungeon levels, completely random generated, themed by their depth.
Even in the original sequels ... D2 was still random, but less so than D1, and D3 is static with just some randomized locations.
