OpenVPN. I can self-host a lot of different things for my own personal internet/cloud thingy - Bitwarden, Pi-Hole, etc; but to actually use it when I'm not home? I either pay for a VPS or I just have a VPN back to my home network. So much becomes possible with that; like I have the contents of my entire NAS at my fingertips even if I'm not home; I can render projects on a VM at home and manage it from my phone, etc.
OpenVPN is, without a doubt, the most important part of my infrastructure.
I think the major issue that people might be concerned about is they might try to use a carrot to get people reliant on their instances.
Then they start making breaking changes and integrating proprietary stuff into it so that it's all much more closed source; and unless you're on an instance that they control, now suddenly a lot of the people who you used to be able to talk to on the Fediverse just can't be interacted with from your instance.
Like, the "World Wide Web" is the primary way people use to access the internet (we're using it now!); but despite the W3C you'll have commercial browsers that just don't play nice with the standards and some websites that wind up, because of that, only working in commercial browsers (Internet Explorer was infamous for this, and apparently Chromium has its issues with this as well). You further see websites that'll attract a huge userbase - using that open standard - and then kind of try to push everyone into using a non-WWW (but still internet!) app; meaning that what once you could have accessed from any web software you're now restricted to one single option that's beholden to the whims of that corporation. That's not to say that any of these is what Facebook will do (I'm not actually concerned that they're going to try to lock it behind an app; it's just an example that I can think of, especially given the recent Reddit drama - Reddit is also trying to kill their mobile site and effectively move people on mobile from the open WWW standard to an app, and have made mobile painful for awhile; and they're not the first site to add a "This site is better in our app" banner to mobile)
Of course, with the Fediverse they'd have a lot harder of a time doing that quickly without the cooperation of a few big instances. We aren't really sure what the people who spoke to them have agreed to, but we do know that they signed NDAs meaning there's something that Facebook doesn't want them to talk about.
Personally if this really was something good that we shouldn't be worried about? They should be able to be transparent about it. They're not, and that's concerning.
Honestly at the end of the day I'm just tired of people trying to make every single cent they can off of me. I want to live my life and have hobbies and talk to people and I'm tired of some greedy asshole taking a look at the tech and creativity that enables it and going "But how can I make money off of that?"
Some things shouldn't be about money.