The Dark Forest question

the droplet killing damn near everyone in just a few minutes.

For me, it was that the droplet was composed of material so dense it was held together by the nuclear strong force.

I think the coolest thing was when Luo Ji is awaken from hibernation in the future and we discover that the whole wallfacer project has been almost completely forgotten, and that Earth's progress in technology and space warfare is actually huge and probably beyond the Trisolarean power.

Total surprise for me, this was a very clever move by the author.