To call Victoria II dense would be a gross understatement. It's a pound cake, with each ingredient -- the tangled politics, vast wars, and simulated economies -- adding greater depth and increasingly complex, interweaving systems. Even for those who consider themselves to be an experienced Europa Universalis player, as I do, it can be a daunting grand strategy title.
The second expansion, Heart of Darkness, overhauls some of these systems, like the naval combat and colonization, while mixing in new elements to make Victoria II the best it's ever been. National newspapers give international events more character in an easier-to-digest fashion, while the Crisis System makes the way powers interact more convincing and tense, also offering more gameplay options to smaller countries.
Underneath the -- sometimes confusing -- menus and colorful historical flavor is a monster of a strategy game, and one which will cause you to come back to the real world ten hours later, wondering how you managed to waste an entire day colonizing, fighting, and trying to please the eternally rowdy masses of your chosen nation.