The expansion adds two new campaigns, 10 new heroes, new swamp tileset (for the world of Dreanor), new soundtrack and several multiplayer maps.
The new campaigns feature ten new heroes for Warcraft, five for the Alliance and five for the Horde. Warcraft II: Tides of Darkness had a few hero units, but these units were only marginally stronger than their basic unit and had no speech of their own. The new heroes had increased unit statistics, making them more powerful than regular units, had their own unit portraits and their own speech. The Alliance heroes were Alleria (Ranger), Danath (Footman), Turalyon (Paladin), Khadgar (Mage) and Kurdran (Gryphon Rider on his mount Sky'ree). The Horde heroes were Grom Hellscream (Grunt), Korgath Bladefist (Grunt), Dentarg (Ogre-Mage), Teron Gorefiend (Death Knight) and Deathwing (Dragon). While regular units of the two armies were balanced, only the spells wielded by spellcaster differed, the heroes differed. The Orcish heroes were more powerful than their human counterparts. The heroes are important to the story and may not be killed during the mission with exception of the final missions in both campaigns, with the exception of Khadgar in the Human campaign.
The amount of missions is less than the Tides of Darkness campaign, however. While both orc and human Tides of Darkness campaigns have 14 missions each, orc and human Beyond the Dark Portal campaigns have 12 missions each. But the increased difficulty of the maps, the new heroes and the interesting scenarios overly compensate for it. Beyond the Dark Portal campaign missions may be found a lot more fun and a lot more difficult than the Tides of Darkness ones.
You can also access the new heroes, new tileset and all the new maps from the updated world editor as well.