I am so afraid for DnD I do not plan enough.
Part of it is because I like to run my games with a plot line encased in a very open world. It's like Skyrim (which I've been playing a lot of these days), but I'm hoping a better received main plot line.
The problem with this is that you never know what your players will latch on to and want to pursue. It's not only the little tidbits you thought would be easy to have your players forget, it's also the players interpretation of the big things.
For example, the last two times I ran a game in my setting, I definitely had players who took the role of working for the King seriously. They had experience and honor and lineages that they wants to make sure came into play. They knew that they were the best of the best for a reason, and it was an honor to be serving the King.
These players feel they are not the best of the best. They have bad habits and not the most impressive histories. Two of them are from a friendly kingdom, and aren't entirely 100% sworn to the king (they already have one). One was busy trying to sell potent potables to the public as a druid. One is busy trying to impregnate every lady he can. And one is merely amazed that he has a warm bed and food provided to him as a courtesy. They know they are not the best of the best, and if anything it makes this same setting feel more desperate. This isn't a position of honor. This is proof that the Kingdom needs to be pitied.
With the same introduction and the same world, the feelings are entirely different. Imagine then the difference between the person who knows everything in the world as "God" so the speak, and the difference between the players who only know what I tell them.
Therefore, considering all the unknowns with the decisions made, you cannot plan enough.