A separate thread in opposition gives you a separate discussion. It does not, however, prevent false conceptions about the degree of support a concept might have in the community for people that only see one of thr two (+) threads.
I do have concerns about false conceptions created by some (+) threads. I'm not saying things need to change, or that I have a better approach to offer - just that I have concerns. When you see a thread with 200 messages supporting a topic and no opposition to it, that creates a perception of unanimous support when - without the plus - there might have been 800 posts opposing the concept. When it comes to something as trivial as a game mechanic discussion, I don't think that is too big of a risk. However, my concerns arise when the issue under discussion has ramifications outside the game.
For example, if someone were to do a + thread with a title saying, "Wyrmwood is a high quality product and we should support them 100%! (+)" and a bunch of people who like Wyrmwood product posted their support for the quality of the products, their stories of use of products, Critical Role's use of their products, etc. ... but we didn't allow any posts that oppose Wyrmwood, including that that address recent news, we might create a false perception that there is no concern in the community for the accusations that have recently been in the news (involving firing someone for reporting a sexual assault, stealing IP, etc.) Potentially, it could even create a perception of support for Wyrmwood by Enworld. What would we think if that thread were used to show that 'a large number of people in the community' doesn't believe the accuser and supports Wyrmwood, and that there were dozens and dozens of posts in support of Wyrmwood without a single opposition posted in the thread?
I am not getting this binary oppositional thing you are trotting out. First off, do you think there is a way to declare a winner on a topic by the number of people that take X side vs. Y side?
A +plus thread about Wyrmwood wouldn't be something about unconditionally supporting them and hey, all you haters jump off. It would be a topic about the quality of the product and pluses and minus of it.
A + thread does not remove all discussion but it does ask that people who want to come in hard say, "Wormwood is the suckiest of the suckiest companies out there and nothing else." to not come and play in this thread. Does a unbending no that shouts down a topic bring any value to the discussion? I have seen a lot of hot button threads end because a group swoops in and shouts everyone down. I personally don't see any value in it.
If you feel that strongly about it, start your own thread where you can go to town on Wormwood. Both threads are going to show up in a search and the rest of us will decide which one we think gives us the best info. You do not get to choose the winner, we do.
Let's say, for the sake of argument, that Wyrmwood has done something illegal or unethical. Let's pretend they get caught substituting cheaper wood materials and not the ones advertised. That would be its own thread and you would not dump it in the already existing + Wyrmwood thread. And there is nothing stopping you from doing just that. Where is the crisis?
Now if the members of the + Wyrmwood thread decides to take up the allegations on their thread and don't allow counter points you could take that to the Mod staff because their topic is drifting outside there own parameters. Trust me the scandal thread is going to bury the + thread anyway.
As for the rest of it, do you really believe the mod staff is not going to come down hard on a thread that is supporting illegal and unethical behavior? Why don't you try this (Actually don't, but this is an example), start a + Thread on NuTSR and the impending litigation with WotC and see what happens. I don't think the topic will last very long or be very popular.
Having a plus thread for a gaming mechanic doesn't seem terribly problematic to me. If someone wants to have a plus thread for talking about how Bards are the best class - more power to them. However, having a plus thread for a concept with ramifications outside the game has higher degrees of risk - and drawing a line to determine when different treatment would be appropriate would be difficult at times.
So you are asking for a comprehensive hard line from the mod staff on what is acceptable? You are expecting them to pre-game all types of + threads that are possible? Nobody has time for that and no one would read that beast of a document. The rules for + threads will evolve as the topics put under them grow and evolve. When something crosses a line or abuses the form, the mods can make a ruling and we all learn together. Right now your are buying trouble we do not own.
Would it be ok to do a (+) thread supporting the use of explicit images in RPG artwork? Or (+) for using historically accurate and detailed depictions of heinous acts of slavery, sexual assault, etc...? Or a (+) thread encouraging DMs to use questionably moral techniques to trick or manipulate their players to help the DM get paid to DM? How would we have felt about a (+) thread that supported the new TSR through the last couple of years?
Has anything like this happened yet? I haven't seen one these type of threads. This in my mind a purely hypothetical position, cue the "Think About the Children!" gif from the Simpsons. There is a simple solution since all of these examples pretty much violate EN World Rules you report it to the mods. The mods take a look and if it is violation of the site rules, down it goes.
Would a non-discussion based 'downvote' tool be a reasonable way for people to show an opposition to a + thread topic without 'trolling up' the discussion?
I am not getting the why you need to have a say in a + thread that you don't agree with or don't like. It is not for you. There are tons of threads I don't care about, don't agree with, and a few I have not liked... I didn't not feel diminished when I didn't comment on them; the world kept on spinning and EN World did not implode. A + thread is asking you to respect the OP and those interested in the topic. If you cannot, move along.