If you don't like listening to a long video, TechPowerUp has a summary over here. Apparently, AMD made quite a number of changes to the CPU design. The new chips promise lower power consumption, lower temperatures, and lower latency:
The added information explains that there is no usage of microbumps - instead, there is a perfect alignment between the bottom layer (with the CCX) and the top layer (the L3 cache) which enables the bonding process to occur naturally via the TSVs (Through Silicon Vias) already present in the silicon, in a zero-gap manner, between both halves of the CPU-cache sandwich. To enable this, AMD flipped the CCX upside down (the core complex now faces the bottom of the chip, instead of the top), shaved 95% of the silicon on top of the upside-down core complexes, and then attaches the 3D V-Cache chips on top of this formation. This also has the added bonus of decreasing the distance between the L3 cache and the CCX (the distance between both in the Z axis is around 1,000 times smaller than if the L3 cache was deployed in the classical X axis), which decreases power consumption, temperatures, and latency, allowing for further increases to system performance.