I'm gonna be a little bit all over the map with this one.
Primal and Shadow Power Sources.
Divine Spheres and Specialty Priests. Having different Priest classes for different settings.
Name Level and Divine Ascension. (With the former also being mechanically important.)
Race-as-Class and Racial Class Limits. These speak to the definition of non-human races.
Not an overall fan of the 4e Cosmology, but Shadowfell/Feywild/Far Realm are the bee's knees.
2e's settings, particularly my beloved Spelljammer, the FR sub-settings, Dark Sun, and Spelljammer.
Also... I wholeheartedly approve of the decision to discontinue Oriental Adventures after 1e and 3e, and assign Monks and Assassins to Psionic and Shadow rather than giving them their own Ki Power Source. But... there are a lot of things from those problematic tomes that I wish had become Core D&D: mundane and magical martial arts for every class, Kensei and Shaman and Shugenja, Nezumi and Tengu and Vanara.
Primal and Shadow Power Sources.
Divine Spheres and Specialty Priests. Having different Priest classes for different settings.
Name Level and Divine Ascension. (With the former also being mechanically important.)
Race-as-Class and Racial Class Limits. These speak to the definition of non-human races.
Not an overall fan of the 4e Cosmology, but Shadowfell/Feywild/Far Realm are the bee's knees.
2e's settings, particularly my beloved Spelljammer, the FR sub-settings, Dark Sun, and Spelljammer.
Also... I wholeheartedly approve of the decision to discontinue Oriental Adventures after 1e and 3e, and assign Monks and Assassins to Psionic and Shadow rather than giving them their own Ki Power Source. But... there are a lot of things from those problematic tomes that I wish had become Core D&D: mundane and magical martial arts for every class, Kensei and Shaman and Shugenja, Nezumi and Tengu and Vanara.