It's weird how Torchlight 2 and Diablo 3 share similar problems as you progress. Both have very intriguing scenarios near the beginning, with a somewhat workable story. Then as you progress, game mechanics just start falling apart.
Secret rooms were super cool in Act 1, but basically disappeared after that, while Diablo 3 had cool environmental hazards to use on enemies that just ceased to exist past Act 1. Act 2 is the extremely long desert act for both games. Act 3 introduces some of the most annoying mobs in the game, for the case of Torchlight, leaping and phase wolves / charging mobs, while Diablo 3 you have spear throwers and birds and tremors. Act 4 in both games basically turned into a large dungeon with no sense of exploration or atmosphere, just a straight jaunt from one point to the next, similar to a huge marathon. Fishing holes just disappear in Act 4, which is a damn shame. (Unless I never ran into any). The stories at this point also fell apart, I didn't even know what was going on in Torchlight 2 at the last boss, and in Diablo 3, it was just an anti-climatic boss.
Then you beat the game, and where are you left? In Torchlight, you have accessibility to maps and new game plus, while Diablo offers you a higher difficulty. It's basically the same shit, new game plus and Nightmare/Hell/Inferno, me running through the same story and map, just with harder monsters. The maps are refreshing I admit, but in the end, I'm just playing to get more loot so I can eventually get stronger and get loot faster, which is what I did in Diablo 3, except that in Torchlight, an open system means that there is no real competition, no real incentive to go on. In Diablo 3, the loot was shit, but when you did get something nice, you could sell it, and it'd fund the rest of your new gear and you knew that you're making progression against other people who had to go through the same loot shithole that was Diablo 3.
Loot is a big one too. Torchlight 2 does give me way more oranges, but most of them are useless to me. Whether they be class specific, or just stat allocated wrong, I pick up a lot of more rares/uniques than I did in Diablo 3. But just because it's orange, doesn't mean it's better. People have complained that there wasn't enough legendaries (before the legendary patch), but it doesn't matter, if legendaries are dropping a lot, and they're shit/useless, then why do I care if more drop. In Diablo 3, I can sell them for a marginal benefit. In TL2, I'm stuck either transmuting them, or vendoring them because they don't serve much purpose. I'm level 55, and my average iLevel is probably close to 40, I'm still sitting on items from level 20-30 because I haven't been able to replace them yet, which is a criticism I hear often for Diablo 3, that you sit on items for long periods of time. It's really no different here, except without an AH, I can't solve the problem, I just have to hope a mob drops me something I can use.
Torchlight 2 does have an overall more rounded system though, I wish D3 would steal the pet concept and implement them into followers, the whole buy potions, sell junk thing is really nice. Enchanting isn't too bad either, same with gemming. Fishing is a nice diversion and enhances the pets, which is always a plus.
However, the biggest thing I've noticed between the two is staying power. A good portion of my friends have already finished the game and quit, as there is no incentive past beating the game. While in Diablo 3's case, people played the game (even though they "hated it") for countless hours because they wanted to reach that end difficulty of Inferno. I don't know why people are more motivated to aim for Inferno over New Game+++, but some are. As is, I played Diablo 3 for 250 hours, and I'm going to probably play Torchlight 2 for 50 hours. Is Torchlight 2 the better game? It might be, but it's certainly sad that Diablo 3 is going to hold my attention longer than Torchlight 2 ever will. Neither game in their current state will ever reach the staying power of Diablo 2 back in it's hey day, but I think that's more an issue of the time Diablo 2 was released, and the increased variety of games and entertainment available to people now.
That doesn't take anything away from Torchlight 2 though, it's a fantastic game and I'm glad I played it, but it certainly wasn't the end all game that people were hyping it up to be. It's more fun than Borderlands 2 at least.