A lot of players, and developers, look at grinding for levels/experience as a lame tactic to increase a game's length. While this is true, in a lot of cases, it is not always absolute. A perfect example would be the Disgaea franchise. With each game the formula gets better but let's look at Disgaea 3.
While it is never actually required to grind to finish the game, grinding levels does make the game easier. The same could be said of almost any game with a leveling mechanic. The difference, however, is that grinding in Disgaea 3 is fun(Well to me it is anyways). You won't find yourself doing the same thing over time and time again. There are so many classes to choose from, different stats to build, different weapons to choose, more abilities then you know what to do with, 'evilties' to exploit, and so much more. The customization is practically limitless.
The thought that soon my Time Mage would have every spell unlocked, my Knight would be able to become a Monk, and later make that Monk a Thief so I can eventually switch it to a Dragoon. I mean how cool would it be to jump around as a Dragoon, but also have the added movement of a Thief? These are the things I drool over and dream about at night. These combination of traits from other classes made me sweaty, thinking about the most potential combinations. To become an all powerful force is a very entertaining idea.
The same methods don't always work. Sometimes your axe just won't cut it and you need to hang back with your magic. Suppose you charged in heedlessly and are now on the brink of death, poisoned. It's gonna take some real good maneuvering to rectify your mistakes. By keeping a keen eye out for traps or items that alter properties around them you can create new advantages over your enemies. So while you are continually grinding fighting these enemies, you're also having an interesting new experience.
I've played all of the RTS games and have spent hours discussing the lore, but when it comes to actually leveling in the game... I just don't want to. I love the grinding to get new equipment. It's such a rush doing all the dynamic instances recently released or farming honor in competitive PvP, but oh Fate Above save me from the leveling. It's always an incredible chore for me to level up characters in that game because I've done it all before.
Regardless of where the quest is at and what the storyline is behind it there is only one thing that happens. Kill this many monsters until you kill them all or get this much of this certain item. Sigh. Just because they look like different monsters does not mean they are. The pigs I fought in Westfall represent the same challenge to me as the giant Mammoths across Northrend.
It's a melee mob. They all react the same way. I know exactly which abilities to use and what the most efficient order is. There is no challenge. Now Disgaea gives a lot of story for the main games battles, but at the same time you still have all of the changing situations. If only leveling in World of Warcraft by doing battlegrounds was more effective. I would level every class to max as I enjoyed the talent trees, gear acquisition, new abilities, and player interaction.