I am an entertaining but potentially ineffective GM
For example: In the first season, the bad guy was Azael; he's the demon associated with scapegoats and sex, so he's got some goat-like features and he encourages sloth and dissipation. He's Fallen, but got stuck in the body of someone who's dying and isn't happy about the situation, since although he can keep them from dying, he can't fix the weakness of his current body all on his own. He needs worshippers then, and power, and preferably a different vessel. He's learned, though, that you might be better off if you craft and prep the vessel first, so you know what you're getting, so he recruited some college undergrads to start worshipping him as he picks one and gets them ready to take over.
Sounds legit, right? It was going really well -- everyone was creeped out at appropriate places, and then they started getting close, so the Fallen ambushed them with summoned demonic constructs. Here, however, is where I went wrong. I combined the stuff Azael is known for (goats and sex) and made anthropomorphic goat demons -- female goat demons -- and then I saw the Addicting Saliva power, which totally worked with his rap sheet, but saliva seemed the wrong choice, so I went with a different body fluid -- milk. Because goats. So I created sexy goat she-demons who lactate addictive milk. Again, each step was reasonable. The end effect, though... less so. My players were creeped out, but also just sort of gobsmacked and have NEVER let me hear the end of it, in a good natured way. :)
This last game, I prepped for a lot of different combats, knowing my players were going to be in different places. There were underwater constructs, and then ghouls on land, and then I wanted the characters in the boat to be attacked, so I chose an air-based threat to punctuate (for me) their opponent's level of control. From the air indicates birds, which has a nice Hitchcockian feel to it. Seagulls are good sea birds, so there's that. I didn't want just regular mind controlled birds, though, so I decided there were five demonic construct seagulls that were tougher and meaner and controlling the flock as cover. Again, spooky, etc., and it was all fine until I went to do sound effects (because I like to describe smells and sounds and sensations in my games) and I made the sound of a gull who got hit by a bullet -- and then the table burst out laughing. Apparently I make good gull sounds, but that's of very little help when all they do is send your players into gales of laughter. And what was worse, I kept doing it, because that was the way they could tell they'd been effective, right? Not better.
So really, I think my issue as a GM comes down to a blind spot regarding context and reactions -- I have a hard time predicting the end effect of what I set up as a GM until my players give me their reactions. The games are always interesting this way, but not always what I intend. Now I just have to decide if that's a bad thing, so long as my players are having fun.