Goddamit, Will! I was sitting with my boys last night thinking about starting this exact topic! And you stole it out from under me!
In all seriousness, this is a constant topic of conversation in my house since we have one boy (Drew - 14 1/2 mo at writing) who will eat anything except cauliflower and another (Max - 3 at writing) who won't touch a vegetable unless it forced in one way or the other. Max isn't a big problem since he'll eat any fruit out there, so he does get enough vitamins.
There are certain foods that I expect to be a problem - onions, garlic, really anything with a strong taste. But we couldn't get Max to eat any vegetables until my wife had the stroke of genius around his third birthday of giving him 3 pieces of whatever veggie Drew was having (seriously, that kid eats broccoli, carrots, peas, beans, plus whatever else is put in front of him) and telling him that he has to eat it because he's "three now and a big boy" (if only that worked for pooping in the potty....).
So I'm highly motivated to see this thread fill up with good advice. Please, coming the guy that has to threaten to get the stick out to force the food down his throat, help Max figure out how to learn to like more vegetables!