Well, in short let's put it this way:
1.23:
Combo System - Many positioning requirements and other factors met (i.e dodge/missed an attack/evade/under x effect etc)
ARR:
Combo System - Very very few positioning requirements that hardly matters outside of Monk.
1.23:
Battle System in General - The game was on an engine not suited for MMOs so it had some core problems, so if you wish to have experienced it, take what we got with ARR now and add depth and more involvement into it ontop of gear actually meaning something:
I'm also in the camp that 1.23's battle system was better and assumed ARR's would have been an extension of it, making it even better.