Well, to start, since you said yourself that you didn't have much experience with Diablo (2), what you should know is that Diablo 2 has both an open and a closed multiplayer option: the open mode lets you use your single-player character online and (technically) allows you to host a modded game, so you can play mods in multiplayer and be exposed to literally anything that's possible within the game's engine; the closed portion stores your character online and requires you to reference your game with the official patch version at all times, so you can only play the unmodified, regulated version of the game. Torchlight 2 is only offering the "open" option; Diablo 3 only offers the "closed" option. Diablo 2's closed option had at least a hundred times the amount of players that the open option did, and that ratio got even wider as the game got older.
The fundamental thing that makes the closed option more compelling than the open option for long-term multiplayer is that allowing players to do whatever they want with their characters instantly and permanently devalues anything legitimate you do with your character. In that sense, for a multiplayer-oriented person like myself, there's not really much incentive to build an amazing character and set so-called "perfect" item goals when I know that the game actually gives players the tools to create this character without ever really playing the game. It's not quite a simple as all that, however. What's more important is that it means that there are any number of other like-minded players that are thinking to themselves that there's no use in valuing anything found in the game because mod tools make it pointless to "compete" (not in the esports kind of way but in the "I'm useful in a co-op game" kind of way) while playing legitimately. As a result, the powerful trade economy that fueled Diablo 2 and kept people playing it a decade after its release (visit a website called d2jsp) will never form; the open nature gives everything, whether it is found legitimately or not, a value of 0 in the macro economy. This is irrelevant to people who will only play single-player or with their friends but it makes players like myself who played Diablo 2 well past its intended life span almost entirely on the basis of accumulating wealth and powerful characters -- of which there are many -- stuck. Yes, mods are nice (I've played a lot of Diablo 2 mods), but they don't have the same long-term appeal.
