A common misstep I see dungeon masters make, myself especially, is to put off their good ideas. They, or rather I, take their craziest, most heroic adventures, monsters, dungeons, what have you and plan to run them sometime down the road. For myself at least, there are several reasons for this, working together to make my campaigns somewhat less than they ought to be.
The first is the idea of a campaign progression. The idea that I start with small, mundane challenges before diving into the realm of the fantastic. This makes sense, on both a storytelling and gameplay level- you need to set up the world before the characters see its extremes, and you need appropriate challenges for low level adventurers. The problem is that I often overdo it. I’ve never run a liche or a rakshasa, never ran an adventure in the lower planes, even though I’ve run more than one campaign that has lasted over a year. I don’t want to throw out my best stuff right away, and often never get to it. Especially since most of my campaigns don’t last more than a year.