I dunno about forgotten, but these are my favorite old console features:
PS3:
Being able to download media off the web browser, like movies and such. Also, using a USB stick to transfer music, movies, saves etc from one PS3 to another.
Being able to rip music CDs to the PS3 and create MP3s out of them.
You can play the MP3s while playing certain games. On PS4, all you get is Spotify.
OG PS3 obviously runs PS2 and PS1 games at higher upscaled resolution, without the need for a physical memory card. All PS3s can run physical PS1 games, but yeah.
Can support (8? Or 6?) DS3 controllers at once vs PS4's 4. This actually makes a difference if you ever wanted to play NHL or NBA2K with that many people locally.
Wii:
Being able to play GameCube games at 480p with component cables (the GCN ones cost more IIRC).
Using the Wavebird on Wii for GCN and certain Wii games. Obviously there are lots of consoles that don't let you use old controllers.
The Virtual Console was a cool service and cheap alternative to paying for physical versions of most old games. Plus, older consoles can have issues connecting to modern TVs.
Problem now is that it seems lots of new TVs don't have component inputs.
PS2:
Made PS1 completely obsolete with its "Smoothing" feature as well as faster loading times for PS1 titles.
PS1:
Could play music cds.
Wii U:
Can play Wii games through HDMI. While not as crisp as component cables (IMO), the plus side is that any TV these days has HDMI input.
Great streaming through the web browser. Able to stream to TV, and surf the web on the Gamepad simultaneously. Use of web sites like Couchtuner and Putlocker. Much superior at browsing web than PS3 or PS4.
Able to run games off of USB sticks.
OG Xbox:
Not much experience with Xbox, but playing music while playing games was a first IIRC.
HDD meant faster loading times and higher storage capacity.
GCN:
Nothing off the top of my head other than having the first official wireless first party controller with Wavebird.
Dreamcast:
First online service. Didn't work for me very well.
The memory card thing.
DS:
Suspend mode was awesome coming from GBA.
Getting bored of this post sorry.