From Reddit:

TL;DR

• Thin client, less to download, faster loading times, helps keep min specs low

• All new physics and aerodynamics engine with much more control for aircraft developers to define aircraft surfaces/geometry to make full use of the flight dynamics.

• Fully simulated hot air balloon physics.

• Massive performance improvements, multi-threaded

• Backwards-compatible (community folder still there)

• Updated systems (electrical, pneumatic, fuel and hydraulic)

• Failure, wear and tear system

• Payload and passenger system overhaul

• Deeper and wider avionics packages

• Enhanced tree diversity

• Improved cliffs

• 3rd tessellated ground materials

• Improved ground details - Screenshot

• Full seasons

• Aurora borealis, tornados, improved storms

• Animals herds and migration

• World live ship traffic

• Accurate live traffic + more real liveries

• Improved vehicle traffic

It's looking pretty good, hope the transition from FS2020 to FS2024 will be a smooth one!