So was the difference between grinding and farming based on if you enjoyed the activity or not?
Not absolutely, but because most people consider grinding to be repetitive activity specifically to earn rewards from that activity, if you're not doing it for the rewards because you just enjoy the activity, that reduces the likelihood you'd see yourself as grinding. Also, because there's a lot of grey area as to what constitutes repetitive activity, if you're enjoying yourself you're likely to lean on the conservative side of judging something as repetitive.
Is street sweeping "grinding?" It depends on whether you see that as repetitive or not. Why is killing a group of spawns in a mission considered doing one thing and not thirty repetitions of one thing? Why is it different if its done in Brickstown instead of a mission? Perception has a lot to do with what someone will call "repetitive" and the degree to which we perceive what we're doing as repetitions of a small unit of activity or sequences of actions to complete a single activity.
And here's the kicker. If your *only* reason for playing MMOs is character progression and earning high powered high level characters, and you don't care about missions or story or plot or even type of enemy or anything else, then in a sense *everything* is a grind and the only question is how much swings of the sword will it take to reach level 80. That's an extreme point of view, but the reason why I said there isn't a definitive definition of "grinding" is that everyone has a slightly different definition that sits on a continuum between "everything is grinding" and "only when everything is exactly precisely the same and the action is forced is it grinding." At no time in the history of gaming did a committee decide to specify where on that continuum the official definition lies. About all we can say is that the median definition appears to be a little subjective: the act of repeating the same or similar activity with the primary intent being earning the rewards associated with that activity. How similar it needs to be depends on who you ask.