So John Funk opened up the topic of grinding this week, a subject near and dear to my RPG playing heart. You should probably go read that now.
As is evidenced in the comments of that post, there are a lot of different definitions of the word "grinding". Everyone agrees that it's "repetitive and boring" gameplay, but there are a lot of different ideas on when you hit that point and how much of it is the designer's fault. For me, the grind sets in when your in-game rewards get to be so far apart that the gameplay is no longer enough to keep you entertained.
One of the things that makes a roleplaying game so addictive is that there are so many rewards and activities. You gain a level, which opens up a new area of the world. In the new area you find a key resource you can use in crafting. Then you craft something that can help you put down tougher opponents. Which helps you complete quests. Which gets you another level. One gain leads to the next. New outfits. New mounts. Better weapons. More quests. New plot points. More money. Achievements. New scenery. New monsters. New abilities. Another reward cookie is always around the corner.
As the game progresses those rewards get further and further apart. The typical MMO is designed to support thousands of hours of playtime, and even with Blizzard's deep pockets it just isn't possible to fill that much time with unique and meaningful rewards every twenty minutes. Soon enough you reach that point where the next reward is one or two hours away. If you keep at it, eventually you'll reach the part of the game where it will require multiple play sessions before you hit your next landmark. Near the end you'll play for several days before you get that next dose of positive feedback, and with your reward will come the knowledge that the next one will take even longer.
Sooner or later you'll have nothing to keep you company but the gameplay, and this is where the typical MMOG falls apart. Because the standard MMO gameplay of killing mobs for XP is about as thrilling as an unsalted rice cake. A bowl of Lucky Charms without the marshmallows. A Pepsi where the ice has melted and it's gone warm and flat. It's bland. Uninteresting. Designed to not be too hard or too confusing for anyone. It's a thin base on which the rest of the game is built. It's filler, and it's how you'll spend most of your playtime.
John made the comparison with Team Fortress 2, and I think it's a really good one because it shows just how hard it is to entertain someone for a thousand hours. My main character in World of Warcraft is just over level 40 and I've put a little over 120 hours into it. I've put about the same number of hours into Team Fortress 2. Yet WoW is starting to feel more than a little stale, and TF2 still feels fresh and vibrant. This shouldn't come as any surprise. Every round of TF2 is different. The changing landscape of players, maps and class mixtures will keep things new.