Torchlight is great fun. If you enjoyed Diablo and Diablo 2 then you will certainly get your money's worth from it.
Diablo 2 is one of my favorite games, but I still only play it with other people. If I ever do play it single player, it's only to work up my character in some way so I can show him off to my friends I play with.
As for online griefers, I never have that problem because I only play with people I know. So I get all the benefits you just described about a limitation
of Torchlight while still retaining full benefits of being free to play with other people.
They say there is a multiplayer game coming from the developers of Torchlight, but from what I've read, it's an entirely different game that you would have to buy separately, plus it's an MMO which probably means monthly fees and those pesky griefers.
Torchlight just sounds too much like an extended demo/proof of concept to me with so few characters/classes and other limitations. They even say that the game was made as a framework to base their MMO off of.
I'm sure it's probably fun, but I still haven't seen or heard any reason why I should choose Torchlight from the myriad other Diablo-esque hack'n'slashers out there that feature multiplayer.
EDIT: Grammar and clarification.