In terms of crunch, Arcana Evolved adds a new PC race (dracha, a dragon-man), gives races the option to take evolved levels, adds the ritual warrior class (and combat rites, mainly for the RW but also for the oathsworn and warmain), adds two champion types (knowledge and justice, I think), adds class progressions up to level 25, and a few spells, including level 10 ones.
In terms of fluff, as another poster mentioned, there is the return of the dragons. The AE hardcover contains all the Diamond Throne setting material in it, plus extra, as it details the Unknown West and the far southern regions in far more detail than DT,
IIRC.
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