Upload & Sell: Off
Here's my take on things. I think there are three primary sources of negative tone:
1. Someone is characterized as a "troll" or that they are "trolling" or something to that effect.
2. Someone is characterized a "fanboy" or "loyalist" or something to that effect.
3. Someone is condescending, dismissive, or ridicules someone else. (And actually, 1 and 2 above are really just specific ways of being condescending or dismissive).
I think that no matter what, one of those three things will always happen. But: just because one of the above things does happen, that doesn't mean the entire thread (or entire forum) is doomed to take on a negative tone. The key is to not react in the same manner.
Just because someone makes a snarky remark, that doesn't mean you have to respond in a snarky fashion. If someone calls you a fanboy, you don't have to call them a troll, or vice versa. If someone is acerbic in their tone, you can respond with a more measured, netural tone.
I'm sure you've heard the phrase "Don't feed the trolls." Well, the ironic thing is everytime that phrase is used, a troll gets fed. Because they are looking for a negative reaction, and you are giving them one. If you feel someone is trolling, you can either ignore them, or you can make a reasoned response illustrating why they are out of line. But, once you engage in an attack by calling them a troll, or once you make a dismissive comment, or once you ridicule them, then you've lost. Because any time you are dismissive, you are perpetuating negativity instead of fighting it.
I put it to the forum: the atmosphere here will follow whatever tone you consistently post with. It is tempting to respond to negativity with more negativity, to sarcasm with more sarcasm. But, if you avoid that, then those occasional negative posts will effectively be marginalized.