Both in terms of setting fluff and game mechanics, what are the differences between the original D&D 3.5 version of Eberron and its D&D 4th edition version?
Eberron changed far less then other settings. The following are just a few changes I can remember off the top of my head:
- Xoriat being moved out of orbit, and placed into the stars beyond.
- Baator being added to the cosmology
- "Siberys" planes becoming part of the Astral Sea. This includes: Daanvi, Irian, Syrania, Lamannia, Mabar, Baator, Shavarath. This is only a broad title, and does not change how the planes function.
- "Eberron" planes now orbit closer to Eberron: Dolurrh, Thelanis, Dal Quor. Dolurrh also acts as the Shadowfell, Thelanis acts as the Feywild.
- "Khyber" planes are considered the Elemental Chaos: Fernia, Risia, Kythri, and the Abyss (Though the Abyss is not an orbiting plane).
- Dragonmarks now come in a single size, and are no longer restricted to certain races by the rules (but still by story). Therefore, you can have a Dragonborn with a Mark of Warding if your DM allows.
- The artificer is far less overpowered, now acting as a pretty good leader, focused on either automations or buffs.
- The maps were rescaled.
- Changelings and Doppelgangers are now the same race.
- Things like Eladrin and the feyspires were written in without disturbing lore.
- The Stillborn were renamed the Skullborn.
Most other changes were just add-ins, very little was taken out, if anything.