Venom: You don't use your username to actually log in, you use your email for that. So why have two usernames (different one on the forums, different one in the game)?
You could also go even further and ask, "You don't use your real life name to actually log in, you use your email for that. So why have two names (different one in real life, different one in the game)?"
I say this because some people on this forum seem to have used their real names as their GOG user name, with no intention to communicate with others using their GOG user name.
SleepyChef: Us gamers are difficult to predict and impossible to please. For no special reason i'd like to be able to rename myself DeadlyRaisin and i hope GOG Galaxy allows me to do that. My login (wheter username or email) should be impossible to change for obvious reasons but my nickname, esp. if it's used in MP should be customisable.
One thing that Steam is really missing are options of privacy. This is something I hope GOG will do better.
Currently, Steam allows you to show everything to the world or nothing at all. I want to share my achievements with my friends. I want to share my online/offline status with people. I want to share screenshots, workshop items, and allow people to post on my user page.
However, I don't want people to know I have 200 hours spent playing Super Unicorn Adventure. I want people to know I am online, but I don't care for them to know what game I am playing and when I am playing it. I don't want people to know who my friends are on my friends list nor do I want them to know what groups I am in. I don't want people to know my account user name, let alone previous multiplayer handles I have used.
It's this lack of choice that really irks me with steam, and the reason why I have my entire Steam profile set to private (because I really have no comfortable choice). We all have different levels of privacy that we want, and Steam has an all or nothing approach. I think the more choices GOG allows, the happier it's users will be.