Weaker characters such as Meliadoul and Agrias were improved while Holy Swordsmen type characters had their abilities altered to compete more with other abilities and normal attacks. Weak or unused items were buffed incredibly while encounters were slightly increased in difficultly (after Chapter One) in order to keep pace with the improved classes.
Another large change in Rebirth was the complete overhaul of the speed system to shut down level up/down to uncap speed. We also made standardized speed growth across classes with speed multipliers being grouped into tiers – one speed for slow classes, one speed for medium speed classes, and another for fast classes. We also reduced access to speed increasing items and increased monster speeds. With a buff to spell speed the end result is that spells are valuable even in the late game rather than just the mid game and monsters get more turns in battles.
Here is a list of all the changes made in Final Fantasy Tactics: Rebirth