It's the ups and downs, the extremes - that you could pull your hair out sometimes and then feel serene and happy 5 minutes later (usually because they are asleep!) No-one can prepare you for the intensity of it, the 24hourness of it.
I smile to myself when friends get pregnant knowing what they have got to come, the times you sit on the floor with your toddler and stare across at your DH also sitting on the floor, it's Sunday, it's 9.30am, you've done breakfast, it's raining so you can't go out, too early for their nap, pre-baby you'd still be in bed or reading the papers with a bacon sandwich, but now, what do you DO with them at that age when they need constant supervision? I've never been on so many visits/walks/for cups of tea with random people, just to fill the hours!
Having said that, it is getting easier as DS gets older, and DH participates more in the entertainment activities (which is just as well with Number 2 on the way) and DS is EXACTLY like having your heart walking around outside of your body, I often say he is my "heart in dungarees". Now I get what absolute and true love really means, and I like has been said here before, would kill/die for him.