The "grand finale fireworks ending" of this Baez-o-gram is lovely: an abelian group object in the category SetsΔop of simplicial sets is just as good as a simplicial object in the category Ab of abelian groups. On the other hand, item H of the next one is stupefyingly beautiful: the result is another category equivalence, and it seems to be saying that both of the above things are just as good as a chain complex. I still haven't figured out why.
