The Danse Macabre is quite a major addition to the line. While other games released after Requiem have become more and more distinct from their Classic version, Requiem had the trouble of being seen as too similar to Masquerade. This also meant that the later game lines have also benefited from the game system becoming more and more well realized.
Thus The Danse Macabre is in some ways Vampire going through it's siblings and pillaging the best bits. The result is a huge tome filled with various bits you could possibly add to your game or use to create a new chronicle that has a distinct new feeling.
Chapter one looks at the Tier system, and how the game works at each tier. Something with have seen used in Hunter the Vigil and Geist the Sin-Eaters. it's discussion with respect to Vampire helps in allowing players and the Storyteller to have a better focus on what their chronicle should be. For each tier we get some sample chronicle/story ideas, which are designed to help look at the specifics of gameplay at each of these tiers. Furthermore, there are great insights given for each of the Clans at each of the tiers and how each Clan has specific issues with each of the tiers. We also get the same sorts of discussions for each of the Covenants. the net result is that we understand the nuances of Clan and Covenant at each tier, and how life can differ for vampires of similar power and age. We are also given 'Rules Shifts', which are options that can be used to further enhance the themes and moods of each tier.
Chapter 1 also adds in new optional merits and powers, plus an interesting series of new merits, called Masquerade and Requiem. These new merits essentially add back into Requiem the equivalent of a Nature and Demeanour system for those familiar with Vampire the Masquerade.
Chapter 2 looks at the Covenants in more depth, and in particular offers new, global, versions of each of the 5 main Covenants, allowing for a setting more akin to Classis Vampire the Masquerade. These escalated versions of the Covenants are given new titles, new positions for their members to fill, new merits and powers. Furthermore, the chapter introduces brand new Covenants, some of which are particularly strange and hideous, there even being one that allows for cross over with Changeling the Lost. Hell there is even one new Covenant that builds upon the idea of the God Machine (remember that in the WoD Core book!)
Chapter 3 is in some ways the sister chapter to Invite Only, and gives more detailed rules for social combat (first seen in Exalted 2nd ed). For me this is the sort of chapter you print out to have on hand to aid in the roleplaying of those scenes where you have many characters talking, and you as the Storyteller just can't do all that roleplaying. This then helps boil the scene down to the key conflicts, be it swaying a jury of vampires, shouting down a rival at a party, or directing a mob.
Further additions in this chapter are Tilts (cool combat scene modifications), how to run a montage for things that take place over long time periods and using Disciplines within those scenes, alternative morality rules, and Banes (a representation of a vampire's distance from humanity as he loses Humanity). We are also given an in depth look at Devotions, how they are created and the trials vampires go through to create them.
Chapter 4 is then a look at how to put a lot of these things together. The chapter is filled with sample chronicles, in a similar way to things such as New Wave Requiem, Mage Noir, and Victorian Lost.
Overall a great book, that will be hard to exhaust, and is ideal for any of the game lines as it can provide new rules and also new antagonists.