i agree with a lot of the points on here already made, about poor role models, repeating the patterns of your own parents, being too involved in your own needs to consider your childrens etc.
Also the physical/mental things that can challenge the best intentioned parent: if you are ill, chronically sleep deprived, depressed, stressed etc that can (not always, some people manage fantastically well under mad levels of difficulty) have unwanted effects too?
I think someone further up made the point about giving children choices in their lives. I think this is really important. Not only does it show respect for their opinions and wants/needs, but it helps them to become aware of making decisions, and the consequences of those decisions. So for example, even though ds is only 2.10 i try to offer him some choices and ask what he would like.(park or beach? noodles or rice for tea?)
But to also make sure he knows some things are non-negotiable and why (you must hold mummy's hand next to the busy road to be safe. If you don't hold my hand, we will have to go back to the car and you won't get to walk).
A lot of parenting that worries me is when the parents don't stick to rules, rather than children. No should mean no (if its for a good reason, not just your convenience.) If mum or dad says no you can't watch tv in the morning only at teatime, then 5 minutes later gives in and turns it on just for peace, what message does this send to the child? I saw this recently and then the parent complained to me that their child never listened to them