Terrain that doesn't suck

One thing I really liked about 4th edition was its focus on terrain.  The combination of a high magic default setting and the intense focus on tactical combat was a very fertile ground for terrain.  I mostly map out combats with just a few lines these days, but every so often I'll create a huge setpiece in roll20.  My favorite types of terrain are actually live creatures, either humans or animals.

Crowds: The best terrain isn't rocks, it's people.  I don't put a person in each square and keep track of them, I just list a huge block as "CROWD," and it all moves together. Crowds are hard to push through (2x movement required), can run away from threats and trample folks, and have opportunities for social encounters mid combat.  If you want to hack folks to death or take hostages you can just snatch them out of the crowd. You can intimidate the crowd to form a path through, or to send them trampling over your enemies.  You can get them on your side and have them throw rocks and bottles.

What happens to all the people when initiative is rolled?  

Animals:  A classic example that I use all the time is your horses in a camp.  A 5 foot aura around the horses where if you enter, you're getting kicked. Consider a bear baiting pit with bear inside, or maybe the rocks over yonder have poisonous snakes within.