Aside from the people who shouldn't be on the internet (those who send DMs to strangers when they get upset, and worse), it's a bit of a double-edged sword; in the case of something where the clear majority is frustrated and disappointed, if you post that you actually liked it, then either the disappointed people will discuss the show with you in that thread and give their input and opinion, which will be - even though hopefully constructive - very negative, and then most of the thread will be negative and it might feel like your opinion is being shot down, or, only other people who feel the same way as you do will post in the thread, in which case it will end up a minority echo chamber.

What I've personally found is that if you post about the thing in a very subjective way, making sure that everyone understands that you're posting your opinion and your own, subjective experience about the thing, using terms such as "personally", "in my opinion", "in my view", "I for one", and so on, then people really can't argue your point, because your point is your own, personal opinion, and you're being very honest about it. You will, of course, end up being downvoted even though downvoting opinions shouldn't really be done on Reddit.

However, if you just post something that seems like an objective statement, such as "this show is amazing", without adding the qualifier "in my opinion" to it, then people will often not take it as a statement of opinion, but rather, as said, an objective statement to be argued. Then they (or, we...I do it sometimes as well), will feel like wanting to debate the point and explain why it isn't amazing.

And, if you'd just want your thread to be an echo chamber for your opinion, where only likeminded people are posting, regardless of what your opinion is, aren't you kind of hoping to gatekeep all the other opinions, at least from posting in your thread?

The vast, vast majority of the people here, who felt betrayed by the finale, aren't looking to gatekeep anyone. It's just a whole bunch of frustration over feeling like something beloved has been mistreated twice now.