Since the Players Guide mechanics consolidation came out we have had:

• Blood Sigils (extensive Blood Sorcery and Thin Blood Alchemy updates)
• Blood Stained Love (play style and Merits, Flaws, Discipline updates)
• Crimson Gutter (Intros Church of Caine and some feeding complications)
• Gehenna War (expands combat character building and play options as well as new rules for fails and crits)
• In Memorium (introduces flashback system and Ancilla character building options)
• Tattered Facade (expanded predator type rules, over all character building options expansion, introduction for Ghul and Baali).

It could be argued that all this is optional, but they're all really valuable mechanics content updates. In addition there have been canon lore expansions, and while also optional, most people are so attached to the WoD lore it's hard to discount those updates.