Changes to original RTW from patch 1.3 (bundled with BI) that I have noticed:

-Load-save bug is gone (this is worth the price of admission all by itself in my book)
-Spears beat cavalry, for real this time. Gaul warband took out my faction leader. Twice.
-fixes to recruitment trees and some unit stats (desert axemen in particular)
-shields seem to be more powerful than their numerical rating would indicate, as archery doesn't seem to be as devastating anymore.
-Horse archer bug is supposedly fixed, but I've not tested it myself