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!