Thirded. This thread started out on the topic of "being kicked" but there are plenty of ethical considerations with the "kicking" side of this as well. Certainly there are times when it's justified ... e.g. if someone goes AFK, displays offensively in party chat, and so on. But the most "gray area case" would be if/when they repeatedly don't (or refuse to) respond to constructive coaching and cooperation from the group. The act of "coaching and cooperation" is thus a precedent and mandatory responsibility of the rest of the group. The gray area comes on the judgement of whether is someone is not performing vs. not listening ... if it's the former but they're clearly responding and trying, I would NOT want to kick them, but it's a judgment call.
It's living out the golden rule. While I'd rather never get kicked ... but if I got kicked because I was clearly failing the group and not able to respond to coaching (e.g. by repeatedly standing in a fire ... it happens!), I'd regret my underperformance but understand the /kick. The many examples above where folks have gotten kicked, seemingly unreasonably but certainly with no forewarning or coaching ... then I'd feel like I was treated unfairly.