The worst is teaching them proper etiquette. Don't slurp your cereal. Do make eye contact with adults when they talk to you. Don't say the words fart or poop at the dinner table. And my personal favorite:
I said, DON'T slurp your FREAKIN' Cereal!
I just know there will come a day when my children's spouses will sit dumbfounded at the table of their eternal companion and say, "Didn't your mother ever teach you not to slurp your cereal?" And I just know they will scratch their heads and think back and back and back and say, "uhh . . . not that I can recall, but she sure was a grump on a log."
The successes seem worth it though. At least temporarily. I can't tell you how excited I was when my kids finally learned how to cook for themselves.