I have a few in particular, but my main one, I feel, is the inability to Keep It Simple, Stupid. No matter how many times I tell myself I'm not going to create some grand, sweeping epic adventure, run wild with world-building, lore, and copious amounts of house rules- I always end up doing it. Everything gets way too complex when I'm sure everyone would have had fun with an adventure as simple as "go to this system of caverns full of monsters. Kill monsters, loot, and return."
Many of the things I want in a campaign world I know are impossible to achieve, but I keep trying anyways. Economies that make sense? Politics? Having the players gather not just personal power, but temporal power, in the form of influence and followers? Interesting NPC's the players want to engage with and care about? Lore that players actually remember?
A campaign that spans for many years, and possibly multiple generations of player characters?
Yeah, I set my expectations way too high, and I always come crashing down to earth.