Diablo II with the Lord of Destruction expansion pack is, without a doubt, the best hack-n-slash game there is.
Truly, the more I think about it the more I loathe Torchlight and wish I had encountered an honest review before spending money on it. It copies Diablo II on a superficial level, but the end result is junk. Diablo's story may not be the greatest, but it's so ridiculously deep that I can't even come close to absorbing it all. When I run into a character, I just known that somewhere there is a 10 page backstory. The world just seems so much more alive. Then there is the music. Diablo 2's music was just incredible and it went a long way toward defining the world and atmosphere. Torchlight's music sounds like a Diablo 2 remix which is just wrong on so many levels. It doesn't fit the art style at all and it sounds essentially like D2's music was ripped out and stuffed in Torchlight (I understand it's the same composer, but you think they could have had him do original work for the game).
Then there is the interface. It's hard to see things, the mouse doesn't cover the screen well or with good accuracy due to higher resolutions and the size of the mouse pointer and the combat just isn't satisfying. The new inventory where everything takes up the same amount of space just encourages hoarding giant piles of useless junk for selling to the shops. In Diablo 2 it was always a good thing to get past the first part of the game where you had to pick up every piece of junk to sell it for cash.
Finally, the sound effects just aren't there. Diablo 2 had very satisfying effects for killing monsters and all the rest. It helped give a visceral action-like feel to a game that is ultimately point and click.
The one good thing I can say about Torchlight is that I like the gem system where you can retrieve either the gem or the item by destroying one or the other. That has potential.
Again, I hope they're smart enough (or relentless enough) to get Diablo 3 right.