And then there's Borderlands. Right from the start, Borderlands throws away the ARPG standards by making the game a shooter. It discarded numerous active skills in favor of one, added a huge diverse array of passive abilities, and gave all of it a robust framework of basic abilities. Every character in Borderlands can use a sniper rifle, throw grenades, and has regenerating shields. Every character in Borderlands has a melee attack. All of them can also use the same gear to augment these elements of their gameplay.
Borderlands discards the conventional wisdom of putting the variable power onto the character and instead projects it onto the gear. In a traditional ARPG, your character provides the options and the gear enhances those options. In Borderlands, your gear provides the options and your character enhances it.
Why is this so special? In Borderlands you can change your playstyle without changing your character simply by using different weapons. Finding a new piece of gear can dramatically change your character's preferred style of fighting. You might go quite a while using close combat weapons, and then suddenly get a sniper rifle drop that mostly obsoletes them (in terms of numbers). Suddenly you're engaging enemies from very far away, picking them off one by one with your new toy.
This frequent change in gameplay breaks up a lot of the monotony of the typical ARPG grind. As you progress later and later, you also start finding legendaries that do much more than just the normal types of attack. I really like how the game smartly realizes that you're used to using the standard kinds of weapons, and starts throwing unique special weapons at you. It's not much different than Diablo 2-style uniques, but the impact on gameplay is larger. Most legendary weapons in Borderlands function quite a bit differently than their normal counterparts. It really helps to spice up the game more.
It also helps that the leveling curve in Borderlands is smoother than in Diablo 2. It's more of a normal transition where Diablo 2 had this really terrible leveling curve (very few people actually hit the level cap) and game designers realized maybe that was bad design. D3 has a very smooth curve, similar to modern MMORPGs like World of Warcraft. It's not really an innovation that Borderlands is smoother. It does help that you get new fun skills frequently along with a steady stream of neat guns, though.
Borderlands really starts to grind when you get to the point where you're only looking for a few guns of a particular type. This is especially true of the first game. You want a Hellfire and it doesn't even matter what level it is. Once you have one you want a better one. It's not the best design, definitely.
Overall though, it serves to be fun and interesting even when other games might have gotten old and stale. Torchlight 2 didn't even last to the end for me, but I'm almost through Borderlands 2 for my third time now (two characters through normal, one through TVHM) and it's still enjoyable. It definitely worked for me.