Torchlight 2 so far feels like Torchlight, but better. It's probably the same engine or similar, but there are more environments, more classes, more pets, a nicer item interface, and a few other changes of that nature. Most of the pros and cons of Torchlight can be extended to TL2.
First of all, the basic framework of Torchlight and the first Diablo are, well, identical. They're designed by a bunch of the same people after all. You choose one of three classes, start in a hub town, and you tackle a dungeon that goes down and only down. Sure, there are sidequests and NPCs and such in the town, but the story is an afterthought and the goal is to loot, level, and advance.
Levels and skills - Torchlight is awfully similar to Diablo II. Lots of clicks, lots of loot, and a ton of skills mappable to hotkeys. You have to go into Torchlight's editor to change the hotkeys if you don't like the defaults (the editor's helpfully provided by its creators for free). Each class has unique skills, but all three classes share passives (i.e. + to armor, + to a certain kind of weapon, etc.) that work similarly to the weapon masteries of the Diablo II Barbarian. Levels and skills are almost the same as in Diablo II - you gain levels and get one skill point for level. All abilities can have up to 10 skill points placed into them, or eventually raised up to 12 via +skills on items (+skill equipment is quite rare). There's also a "Fame" stat that works parallel to levels. Every time you kill a boss or a Champion-level monster you gain fame, and every level of Fame gives you another skill point.
Loot is great - hundreds of items, with the best ones typically being high-end sets and uniques. You can also continually enchant items to make them stronger, similar to how the smith in Act I of Diablo II can add enchantments to an item with the Horadric Malus. Your pets make loot even better. Each Torchlight character comes with a pet of your choosing (no difference between them other than appearance, but this might be different in TL2) and your pet represents a second inventory that you can use. If you want to sell off a bunch of your crap without going back to town, you can send your pet up and in a minute or to he'll sell everything in his inventory and bring back the profit. It's possible to beat the game, with plenty of loot and gold, without ever visiting town, but I wouldn't recommend this because sidequest rewards in town can be pretty good.
Combat is fast and crisp, just like the first impressions linked above indicates. Monsters are all meatbags, though, with nothing as challenging as elemental immunities or really brutal auras like Diablo II bosses or even environmental tricks like some Diablo III bosses. You just power through them, and they aren't difficult. Overall, Torchlight is much easier than Diablo I or II (I've only played a little of III) but the final few floors of the main Torchlight dungeon (which is 35 floors in all) are a significan step up in difficulty. Once you beat the main game, you can challenge an endless dungeon that can get pretty tough. Good for achievement hunters or people that want to max out a powerful item set.
Visuals are smooth, bright, and colorful (more like D III than Diablo II) and the visuals are cartoony. Some people compare it to WoW, which I think is unfair, but Torchlight is a lot brighter than Diablo II. It also goes for a steampunk fantasy setting instead of a gothic fantasy setting, and Torchlight enemies include goblins, trolls, golems, lizard-men, rat-men, spiders, and small dragons in addition to Diablo classic undead and demons. Basically Torchlight has much greater enemy variety than Diablo, but is a little lacking in environment variety. That 35-floor dungeon changes dungeon type every 4-5 floors (kinda like in Diablo how the dungeon changed every 4 floors), but I'd say that Diablo II has a slightly better environment variety, especially since Diablo II is a longer game. You can beat Torchlight in a few hours if you rush, but it's probably an 8-10 hour game if you go at a normal pace.
But hey, do you want me to just give you a copy of Torchlight? If you preorder TL2 on Steam you get Torchlight for free, and I have a spare copy from that promo. I can just give it to you and you can try it out for yourself. Most of my friends already have the original because, well, I kinda made it rain on my friends' list during the winter sale when Torchlight cost $3.75. If you want a free copy of Torchlight, just PM me on this forum with your Steam ID and I can send it your way after I get home from work (about to leave for work right now).