The worst steam achievement issues I've seen is them completely not working. GOG is smart enough to know a large number of people want the achievements. And when people buy a game claiming to have them only to find them not working with zero support from the distributor on getting them working... that is a problem. I suspect GOG will work better with the devs on reported problems than competitors do. Steam seems to list a game and then they are done with it no matter what happens. GOG has proven to go far beyond that, so I expect the same will be true with this feature. So I'm not worried about the OP concerns as GOG has done a great job at not duplicating many of the competitors mistakes.

I dunno, I think people will take a broken achievement over let's say... achievement for having one of the kickstarter backer awards. Saw that one recently. 100% achievement hopes ruined unless you were a backer.

DO IT! :D
Dammit, you just made me remember Mercenary Kings you monster. So much potential. :'(

Yikes. That one is a bit rough. I know some are tied to expansions and even DLC as well. How long till we get "spend $1,000.00 real cash in the store"... or do we have that already? Thanks to a few game's outrageous achievement implementations, I've learned to set my completionist tendencies aside and just go for the reasonable ones. I saw one for opening the game over 100 times and another that required a thousand hours of in game time. Yeah, they suck, and I'm learning to ignore those while still enjoying the others that are well done. And since some people enjoy the achievements that I consider outrageous, I think that is the perfect compromise. Do what I find fun and leave others to do what they find fun. Options, options, options!.

That may even help some people with lighter OCD tendencies, "you cannot get this, so you have to learn to deal with the number not being perfect".

Given gog's twisted mentality, I suspect there is an achievement for that.

One feature I want is the ability to reset or delete achievements.

Heck, I should be able to reset everything. I should be able to reset my entire Galaxy profile back to Day 0, or even to delete my Galaxy profile completely and create a new one.

This. I like using achievements as progress markers but on Steam after the first time that doesn't work anymore.

Would be nice if it kept track, like 'First achieved on [date]', 'Achieved X times', Last achieved on [date] etc

Hopefully GOG thinks in its own way and not follow the rest of the sheep thinking.
Thank god for that.
Plenty of people don't want them & feel they are unnecessary for the game experience (yeah sure steamers (sorry!) will say otherwise but that's what they are *used* to)). Besides as others may have mentioned - you will get the inevitable "My achievement for <whatever> in <insert game title> didn't come up, please fix it" or the like (really kind of petty imho)

I just demand a few things in my achievements:

1) Points for getting past level 1
2) Alert me to massive story plot points just as a cutscene rolls -- like "Achievement Unlocked: YOU are the main villain you've been fighting!"

If it has those, then I'll be satisfied.

Sorry, I left a typo on the message, I meant gameplay time.
I HATE the way Steam tells everybody how much time you´ve been playing a game. Totally lame.
I don´t care about achievements, I only care about my privacy.

Even though I have no doubt that Galaxy will have a 'privacy' option so the public won't see your stuff, your choice there if that's important to you is simple. Don't use Galaxy while playing your games.

Use of the client would then depend on if you're fine with GOG collecting data like which games you play and for how long etc, but just don't want to have it publicly visible, or whether you don't even want GOG/Galaxy tracking it.