I have owned a Nexus and I can't believe what I'm hearing (pun intended)!
Samsung "did this" because it's what the majority of people wanted. The reason the volume for notifications is separated from ringtone volume is because the Nexus volumes could not be separated unless you were running a custom ROM! And the reason your notification sound is too loud is because Zedge sounds are known for being louder!
Oh, yeah, and the reason your notification volume was "subtle" on your Nexus is BECAUSE THE VOLUME ON THAT PHONE SUCKED! Go to the GNex forum and you will lose count of all the people complaining about that