Depending on what "original" you played, it may have had the expansions bundled together - Heroes 3 complete came out in 2000.

Features added in expansions:

* Conflux

* Can choose to buy unupgraded creatures from an upgraded dwelling

* Can garrison mines

* New Heros

* Combination Artifacts

* Random Map generator

* Magical terrains

The Random Map generator is the biggest one, likely.

Can re-visit a place by pressing space bar (not present in 1.0, so I have heard)