Since 2012:
- Linux support - I joined around 2013 because of that
- Proton - massive Linux compatibility upgrade
- Steam Input - along with big picture mode and whatnot
- SteamVR
- hidden games
- cart improvements
- mobile app improvements, along with MFA
- collections
That's just what I remember off the top of my head. I'm sure there's more that I just don't care about.