RPGs in general are renowned for grinding, most of the offline ones manage to fit the grinding in well with the story, so theres just enough gameplay and just enough story to not annoy you. Plus they place challenges in the bosses. There are exceptions however, Shin Megami Tensei, Dragon Quest and Blue Dragon namely. Megaten games have a really good story so it does slightly balance it out, but still can get annoyingly monotonous. The other two however, the storys didn't grip me one bit.
MMORPGs however, doesn't have that in depth story to balance it out, instead you end up doing the same quests, killing the same mobs over and over again, and the only thing you get out of it is some pretty coloured Pixels which you'll forget about not long after. The only reason WoW managed to keep me long enough was because I had a few mates online(The rest of the population were the extremes unfortunately, either nooby or elitist).