In no particular order...
1. There will be no time for you once they are born.
2. The worrying is constant.
3. You will be wishing they learn to talk ASAP so that they can stop screaming at you, and then you will be wishing they would just shut up for a moment.
4. It will be YEARS before you get any real sleep that doesn't involve waking up in the middle of the night multiple times.
5. You will often be covered in puke or poo, and sometimes even pee.
6. Your house will never be clean again - give it up.
7. Breastfeeding is much harder than pregnancy and labor.
8. The guy named Genetics is such a jokester - you never know what your gonna get. Your kids can be a lot like the family member you try to avoid.
9. Times will be tough financially especially when they are under five - someone's gotta watch them.
10. You will never love anyone like you love your children even if some days all you want to do is throw them out the window. (figuratively, of course.)