In my opinion the "list" mode when showing the games would be more usable if it was clearly showing which games are currently installed: currently I'm sorting them by activity but there isn't a clear way to separate the installed one from the others.

EDIT: I'm obviously talking about the "All games" listing, I know there is an "Installed" one but in Steam I've always used the list with all owned games.

What about making a "All installed games" listing?

I agree on the indicator issue. I have a long list of games, and many of them installed. I cant memorize them all, so it would be nice to have some sort of indicator in the "All games list" to show wich ones i have already installed.

Sometimes im looking through the list thinking, "hey, thats a game i might play now, lets install it", only to find out that i already installed it a while ago.

I know, its kind of a first world problem, but its rather annoying and should be easy to implement.

I back that. Recently I had to switch between both lists to check which games I had installed. To go inteo every details pane is a bit cumbersome. A green frame (or any color) around the title, or an additional "list mode" with a table (Game Name - Release Year - Genre - Platform - installed?) would be nice.

I like the way Epic launcher shows them, although it is probably too extreme. The uninstalled games are completely Grey and the installed ones colored. Something between could be ideal.

FWIW, you can choose "Group By: Installed" and then collapse down the installed grouping. Not as elegant as a proper indicator, but hopefully it can help for now.