Apart from cruelty and neglect...
Parenting is a 24/7/365 thing that doesn't end at the 18th birthday mark! While everybody may see different reasons to call somebody a bad parent, it is really the lack of effort you put in it.
If you did the best you can and your kids turned out aweful, at least you tried! I know a lot of kids that turned out great despite parents that weren't involved at all.
If you want to put it in a few short words: Lack of Effort!