Selling housing plots: Is this really that negative? I mean it is annoying for sure, but it is a direct by-product of the economy that SE has created. Best way to address this is to not have housing be open in wards, make it instanced, and give everyone a house. Otherwise, you'll get this.
Abusing the Amazon promotion: I am not sure how negative this is either. It doesn't really impact anyone but the player, Square and Amazon. I doubt either Amazon nor Square are losing sleep over this. However, if they wanted to address it, all they would have to do is set it so your code only applies once the sale is final (i.e. shipped/charged), or have the bonus items be directly removed from an account if the player refunds. As a community, we can't do anything about this and I don't really think we should either - (it would simply create an even more negative environment)
Insulting one another - this is a pretty big rarity from my perspective. I could count on one hand how many times in my 3000 hours of playtime that I have witnessed direct insults. I have, however, noticed a lot of people assumed they are being insulted when they are not. There is a possibility that is happening to you, or that you are the root of said insults (i.e. inadvertently, or purposefully, provoking others).
Being rude in dungeons - Again this is pretty rare from my experiences. However, I have seen it happen. Usually, I just ignore it and move on - maybe rant to my friends about "that whm in my expert" to vent etc. It's usually nothing huge, and can easily be ignored. It's rare enough that I wouldn't say it is a community problem that needs dire attention.
Mentors: I have very little opinion on this. From what I have seen as an outsider, the entire mentorship program is a joke. Though, I thought since the announcement that it seemed like a poorly designed system, so I never took part. So, again, I do not have much opinion.
I'd say the biggest negative impact on our community are threads on reddit and the forum calling out on the negativity in the game (i.e. this one, stories etc.) which make it seem like the world is much more negative than it actually is. This makes it feel more negative and spreads that feeling.
For example, people read stories of how toxic Weeping City is, then queue up for it and expect a very negative experience - they refuse to tell anyone they are new and get tips etc. This then leads to wipes and frustrations and people don't know what is going on. Conversely, the alliance I ran Weeping City with on my first run was incredibly friendly (I was expecting it to be toxic after reading so many threads on it). Everyone was laughing and having a good time and we all offered friendly advice to improve. One alliance in particular kept dying over and over and refused to answer/speak up when we tried to address it for 3 wipes straight. Finally one person said something, we helped them, and cleared on our next try.
I guess, if we want the community to be less toxic, we can start by not assuming they are toxic.