Diablo 3 is not a game. It's a casino simulator backed by the Real Money Auction House, and therefore Serious Business (TM). People still playing that game aren't playing it because it's fun- they're playing it because they're addicted to the idea that they might be able to find a piece of gear worth $$$$ and sell it on the auction house.
Everything about Diablo 3 is just wrong. The game itself is horrible, the mechanics are beyond stupid (if someone swings an axe at you and you move out of the way, it's still considered a hit). Blizzard has repeatedly gone on record to tell the players how they should be playing the game (due to the amount of complaints that were raised), and that it's our fault for not having enough fun since "we're playing it wrong". This is not how game developers are supposed to act. Everything about Diablo 3 is designed to keep you in game for as long as possible and make Blizzard as much money as possible with little regard to the actual player experience.
Every single person I know who owns Diablo 3 has quit playing it because of the ******** mechanics where the game simply decides to kill you so you have to go and buy better gear off the Auction House (with in-game gold or real world dollars). I fired it up a few weeks ago to play for a few hours (having previously shelved it) and simply landed up getting frustrated with it again. Does this mean I'm a crappy player? Sure, maybe. That doesn't change the fact that you basically have to dedicate your waking life to Diablo 3 in order to get anywhere reasonable with it, and even then all you're going to land up doing is farming.
Torchlight 2 is the exact opposite of Diablo 3.
It's fun and enjoyable. I really have nothing else to say about it other then that. Runic did a very good job making it an enjoyable experience, and it's worth the time to play (unlike Diablo 3, which I regret ever purchasing and wasting my time in). None of my friends bother with Diablo 3 anymore, but I still see them in Torchlight 2 on occasion. We'll get into a game together and go have fun, and that's all there is to it. Just fun.
So, really, it boils down to this:
Do you want to "have fun" or take up a second job farming gear?
If you want to have fun, Torchlight 2 is just that. If you want to endlessly farm for gear and the incomprehensibly tiny chance that you might find something worth a few dollars on the Real Money Auction House, then Diablo 3 is the perfect game for you. Just don't expect to have any fun doing it though.
-SC