Let's be objective; for it's time, the game is great but did it really do anything new?
The only notable thing is, Warcraft 2 adds a shallow system of navel and aerial units. That's it. The sides are still carbon copies of each other (aside from the spells, only available toward the end anyway), and the strategy is still build a base and a large group while fending off the enemy and then just charge their base.
If you break it down, that pretty much RTS summed up during this era, no different for Warcraft. The missions all play the same for the most part. To further hurt the game, the lore isn't improved at all from the first game as it's just a rehash of good vs. evil, with no interesting characters thrown in. Oh, no Lothar died! Probably one of the most unmoving deaths in a game, especially since he was not that interesting in the first game.
Having played this game for a long time since it's initial release, it is best to leave nostalgia aside and look at the game as it is: it's just okay at best.