I still play the original I bought in 94 (now throguh DOSBox). The features that makes it so good are:
The variety of well diferentiated races to choose from, each requiring a different playing style. Each race has its own troops, in adition to the standard ones, that carried race advantages(one race can fly, other swin,...).
The customization of your character: you can choose any degrees of magic school specialization or perks.
The combat system, which added tactical (turn-based) battles to the grand strategy of the rest of the game.
The AI is a weak, and there are some units that are too powerfull and breaks the balance, allowing an easy win (All death mage with an early wraith summoning spell, anyone?)