I consider it "grinding" if you're doing a repetitive task in lieu of actually advancing the game in some way. Doing a sidequest because it's fun and offers an interesting task(A special dungeon, an interesting side-story, something) is fine. Doing a sidequest because you need like 1000 more experience to level and if you have to kill 40 monsters you may as well have a quest to do it is not fine.
I haven't played GW2(on my todo), but a lot of free-to-play games tend to have a lot of grind to them, even if it's quested grind. Kill 30 of a given enemy, collect 15 of their special drop, then kill an associated "boss" monster before moving to the next "tier" of enemy was a pattern I saw in one game. I haven't experienced this in many single-player games, really. At least not on purpose. Dragon's Dogma was a little grindy, though it only rarely felt so because the combat was the main purpose of the game. Skyrim generally didn't feel like grinding because there was usually SOME kind of story behind a given dungeon, even if the story was just an excuse to go through yet another horde of Draughr. I could see where it could be seen as grindy, though.
I haven't played GW2(on my todo), but a lot of free-to-play games tend to have a lot of grind to them, even if it's quested grind. Kill 30 of a given enemy, collect 15 of their special drop, then kill an associated "boss" monster before moving to the next "tier" of enemy was a pattern I saw in one game. I haven't experienced this in many single-player games, really. At least not on purpose. Dragon's Dogma was a little grindy, though it only rarely felt so because the combat was the main purpose of the game. Skyrim generally didn't feel like grinding because there was usually SOME kind of story behind a given dungeon, even if the story was just an excuse to go through yet another horde of Draughr. I could see where it could be seen as grindy, though.