zeogold: I already know I'm going to get a flood of downvotes for this, so let me just go ahead and save most of you the reading:
As much as it pains me to say this, I'm in favor of this decision.
You done? Good, now go ahead and just push the -1 button and move along. I already know about half of you would prefer to go off that alone, say I'm advocating for violations of privacy, and not bother reading what I'm actually saying, so get it out of your system. As for those of you who actually can be bothered reading walls of text, here's why:

First off, let me say that GOG, to me, is not the "friend" that I imagine it's felt like to most of you. As I'm sure I've said in an open letter or two before, I never had that feeling of "Wow, here's a company that finally caters to me!" like most of you have felt. The reason I support them is the DRM-free, and that's all that matters to me in regards to the company. It's the reason I love the store and the reason I want to see it grow, for the DRM-free. Coming from this perspective, I think this move is for the best in a sick sort of ends-justifies-the-means manner. Although I dislike the fact that it's removing privacy, this is something, as has already been pointed out, the majority of the customer base has been asking for, and for ages. It's something that will hugely boost the social aspect of GOG and, as a result, get more customers.

"But this is a violation of privacy!" you cry. "We should have everything private by default!" I disagree, and I think this action would be GOG shooting themselves in the foot if they did it. Having used Steam myself for a while, I know what it's like. Steam has had everything public about profiles by default for ages with no real complaints. In fact, now that the defaults recently changed, I've heard more dislike for the change to have things private by default, and I can actually see why. I'm against it myself, in fact. Normally on Steam, you can hit up people you meet in games, check out what kind of stuff they play, what kind of groups they're in, etc., and connect. If you want to keep hidden, it's fine, just change your settings. By hiding certain things by default (like libraries or wishlists), however, it removes a lot of that. When you'd normally check somebody out and see if they're worth adding as a friend, you instead see that it's all hidden and decide not to bother. Psychologically, it's a bit like ringing a doorbell where the lights are all off inside, you just don't want to annoy people. I imagine it would be a lot of the same way if GOG makes profiles private by default. While they'd be protecting privacy, they'd massively miss out on that social boost they're looking for. Consumers are usually stupid and, instead of exploring all the options, will just keep whatever default they're handed with and barely look into it. If everybody's private by default, the connections that GOG is looking for won't be made as rapidly. If everybody's public by default, it'll be way easier.

"Steam!" you cry, frothing at the mouth, immediately rushing to leave a nasty response in regards to "Steam is exactly what we DON'T want to be like!" (which will get "high rated" within 30 minutes or less) To which I say you're wrong. I always hear people complain about "Steamification" of GOG as some harbinger of doom, like it's the complete antithesis to everything GOG stands for. I disagree with this notion and think it's painting far too broad of a brush. I don't think there's anything wrong with looking more like Steam. Steam itself is great. It's a handy client with cool, useful stuff. It's got a solid social aspect. People love using it. What sucks about Steam is the DRM. The fact you NEED to use the client is why people hate it. GOG gives us the opportunity to say no to their client (>inb4 snarky comment about the offline Galaxy installers, I know, I hate them too, but I'm talking in general here), and is also giving us an opportunity to opt out of this as well by letting us change our privacy settings.

At this point, some of you have probably gone "But Zeo! These are two different things!" And yeah, I know. It's true, this isn't a matter of, like Steam, new people signing up and getting handed lack of privacy, it's more a matter of existing users getting hit with "Whoops, surprise, everybody can see your library now!" I get it, and agree it's bad. However, I suspect that the people who'll get hit with this kind of nasty surprise will be far and few between. Unlike the trickery with slipping Galaxy into the installers (which I'm still upset about to this day), I suspect, unless GOG somehow MASSIVELY screws this up, this is going to be a HUGE announcement. Heck, a couple of gaming journalism publications have already picked up on it, even. The vast majority of people will likely know all about this, and elcook has even been kind enough to tell us here on the forum (the population who'd care about this the most) in advance.

So, in summary, essentially I'm in favor of this because I believe this move will bring more customers to GOG, which they always desperately need. Again, remember that I see GOG as little more than "the #1 DRM-free store out there" and support them primarily from this point. I believe that this move will be popular enough that it'll boost the userbase considerably. Whenever the userbase of GOG is boosted, that's good news for DRM-free. That's more people who use (or even know about) DRM-free, and, in turn, more publishers who see DRM-free as a viable option. Again, I hate the fact that this means the privacy of existing users will potentially be compromised, but in my eyes, they've given enough warning and it's worth it. They've told us it's coming and it'll presumably be a big deal when it gets here rather than an on-the-sly secret. I'd rather have a few hundred disgruntled users if it means the store gets several thousand new buyers.

Now, understand that you may consider me biased to a degree. I'm younger than most of you. I use Steam a lot. I was never a "hardcore" GOG user to the point where I've considered myself to have great loyalty to the site (where my loyalty lies is in the promotion of DRM-free). I'm aware that there seem to be some European laws that potentially conflict with what's happening, which I'm not at all informed about, nor do I wish to argue about, I'm merely explaining how I feel about the matter. After a lot of back-and-forthing and being on the fence about how I feel about this choice, I've come to the conclusion that it's for the best. I'm giving my opinion on it just as the rest of you are, and you're free to hate me all you want for it.