I am new to android and just by going into Google Play I can see every app my friends downloaded. I am really feeling uncomfortable about this since I don't like to feel exposed.
Does anyone knows how to block this "feature"?
I don't think that there is any trick available which you can use to hide the apps downloaded from your friends until or unless you unfriend your friends over your G+ account.
But I have got another trick for you which is going to work for you like whenever you download app, try getting the .apk file downloaded from third party sites say I want to download whatsapp and don't want my friends to see. I will download whatsapp.apk file directly from the Whatsapp website and then I can install the apk file independently and my friends will never know that I have downloaded whatsapp, just make sure you don't get the apps from PlayStore and it will not be shown!
You can hide your game activity. Go to your profile and click on edit. There will be checkboxes for "People to see your gamer ID through your Google+ name" and and another to "Hide your gaming activity". Uncheck them, and you're good to go!