Just my initial thoughts here... I ordered my first Model 3 on March 31, 2016 and received it in February 2018. Just got my new M3P today, same day I ordered my very first Tesla 5 years later 
Big differences:
* Speed and performance ... The M3P is nutty
* Seats in the 2021 M3P are WAY plusher than my 2018 seats, both front and back
* Headlights are better on the 2021 car
* Suspension somehow feels softer on the 2021 M3P, wasn't expecting this at all (could also be the newer seats)
* Overall ride is noticeably quieter on the 2021 vs. the 2018. Significantly less air/tire noise which I also didn't expect since the new car has the PZero tires and Uberturbine wheels
* Panel gaps are more consistent on my 2018 (go figure)
* UI is different; my 2018 had HW 2.5 and didn't display stuff like stop signs and trash cans etc.
* Enhanced autopilot on my 2018 is more capable (also go figure). I was surprised the "new" autopilot doesn't do as much, probably trying to get me to upgrade to the FSD
* Power trunk! Not sure why I like this one so much
Not so big differences:
* Center console redesign, not sure if I like the newer one better or not yet
* I got an error no matter what USB drive I used in my 2018 that said it was dropping video, the new one came with a USB drive already installed (no errors yet)
* Black trim looks pretty awesome on the 2021 car, the chrome on my old car looks... well old.
That's it for now. I love the new car and plan to keep it all stock for now.
Big differences:
* Speed and performance ... The M3P is nutty
* Seats in the 2021 M3P are WAY plusher than my 2018 seats, both front and back
* Headlights are better on the 2021 car
* Suspension somehow feels softer on the 2021 M3P, wasn't expecting this at all (could also be the newer seats)
* Overall ride is noticeably quieter on the 2021 vs. the 2018. Significantly less air/tire noise which I also didn't expect since the new car has the PZero tires and Uberturbine wheels
* Panel gaps are more consistent on my 2018 (go figure)
* UI is different; my 2018 had HW 2.5 and didn't display stuff like stop signs and trash cans etc.
* Enhanced autopilot on my 2018 is more capable (also go figure). I was surprised the "new" autopilot doesn't do as much, probably trying to get me to upgrade to the FSD
* Power trunk! Not sure why I like this one so much
Not so big differences:
* Center console redesign, not sure if I like the newer one better or not yet
* I got an error no matter what USB drive I used in my 2018 that said it was dropping video, the new one came with a USB drive already installed (no errors yet)
* Black trim looks pretty awesome on the 2021 car, the chrome on my old car looks... well old.
That's it for now. I love the new car and plan to keep it all stock for now.