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!