These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Eternal Sunshine Of The Spotless Mind
Crowning Music of Awesome: Beck's cover of "Everybody's Got to Learn Sometime", which plays over the ending as Joel and Clem chase each other in the snow.
Fridge Brilliance: Clementine's reasons for breaking up with Joel are how he never opened up to her. In trying to keep the memory of her he puts her in all the dark places he never let her into.
Also, of course Joel wants to keep his memories of Clementine, the procedure erases the most recent memories first. As the movie goes on, Joel probably forgets that he broke up with Clementine at all.
Fridge Horror: When Joel initially meets Clementine, he is well aware of the song frequently associated with her name. As a result of the procedure, he has no memory of it, despite its memory only be associated, not originated, with her. If details as insignificant as songs are being lost because of the mind wipe, what else were people losing? There are also a vast myriad of ways this procedure could be used in incredibly unethical ways - implied by Howard's pressuring Mary into having it done after their affair.
A little one concerning the song: it is stated that Joel's favorite toy during his childhood was a Huckleberry Hound doll. Maybe not horror, but having the memory of your favorite thing as a kid is still sad when you think about it.
From the deleted scenes, Howard pressuring Mary into getting an abortion during their affair explains why his wife's reaction to the resurgence of Mary's affections after the wipe was "Don't be a monster. Tell the girl."
Rewatch Bonus: The first portion of the movie makes a lot more sense after watching the movie a first time.
Tear Jerker: Clem and Joel's first meeting and Joel's last goodbye to her memory.