its not the same game as diablo 2, if you're going to compare diablo 2 and diablo 3, there will be big differences, some feel they are worse, but I dunno, its just different.
Alot of things have been simplified, such as you don't assign stats anymore, you can only use 6 skills at once, you have runes that can change a skill slightly, which does make for a few combinations that are useful.
Theres health globes that drop randomly from monsters, it changes the general tactics of battle quite massively. My wizard got extra arcane power and health from the globes, so playing on HC, it was sometimes prudent to go straight through mobs, to get the globe and escape.
Also, its in line with the likes of current FPS's in relation to regen speeds. Mana regens incredibly quick, and health globes make HP go up quite quickly.
Also, skills are a big mix of defensive and offensive skills. Much more than Diablo 2. When I'm on HC, my wizard has 4 defensive abilities, and 2 attacking (1 AoE, 1 Single target) to me, it feels more tactical, I find myself kiting much more and really paying attention to where I'm running. Although from what I read, most people say its too simple. I dunno.
Oh and items, you get alot of them, the AH has made trading so much easier, so 99% of stuff that drops is worthless now. Aparently the softcore AH is silly, you get loads of items for cheap. HC is a bit more stable, since people die and items dissapear.
TL;DR - Its a different game, it make look the same, but it feels very different. Different isn't bad though