You still have to destroy everything to win a mission, even when the opposition has only useless buildings and no gold left. But then this is a scenario pack, not a new version of the game. For multiplayer play this is no big deal, but multiplayer fans might as well not bother with Beyond The Dark Portal and instead build their own maps with the excellent Map Editor from the original game. Beyond The Dark Portal does seem to encourage you to be a bit more adventurous - rather than letting you sit back and defend, some missions have multiple mines which are worth going after. Most tactics remain unchanged.
Catapults can still pick off enemy towers without drawing much opposition out to counter attack.
The lack of new units or AI tricks means you don't have to learn new tactics, bar when and where to use your heroes. This is a plus or minus depending on how you look at it. For the cheap price, the two campaigns are good value, but at the same time there's little new here. The heroes are a neat addition, but the only addition. Or you can simply make your own new scenarios with the Map Editor.