A lot of people dislike it for the privacy nightmare that it is and feel the threat of an EEE attack. This will also probably not be the last time that a big corporation will insert itself in the Fediverse.
However, people also say that it will help get ActivityPub and the Fediverse go more mainstream and say that corporations don't have that much influence on the Fediverse since people are in control of their own servers.
What a lot of posts have in common is that they want some kind of action to be taken, whether it'd be mass defederating from Threads, or accept them in some way that does not harm the Fediverse as much.
Nothing. If people and/or communities coming in through Threads are engaging in good faith, cool, more nice folks to have a community with.
People/communities engaging in bad faith get blocked/defederated as is already common practice (and seems to be working outstandingly already, looking at average quality of posts and discourse "here" as compared to the "big platforms").
When Meta/Threads is hosting communities I like to see/be a part of, I'll figure out how to subscribe/integrate those. Besides that, they're free and welcome to run echo chambers in their own instances and communities, I don't see how any of that would ever show up on my feed.