This is going to be a hard one to explain because on a fundamental level I am crazy for thinking this...but to me it makes sense. My Least Favorite Monster Overall is the Dinosaurs.
Dinosaurs have been a part of D&D and pulp fiction (from which much of D&D has sprung) but I just can't get myself to like it. First I was never the dinosaur-o-phile like so many other kids that also liked fantasy and science-fiction while growing up. Second, dinosaurs are too real for my fantasy...which makes little sense, but does in a round-about sort of way. Let me explain where I am coming from on this...
I will use wolves and bears are combatants in my D&D games because they are real hazards that we all can understand. That would imply that I would also use dinosaurs as they are also real and once were real hazards. Except they weren't, at least not to humans. And we know so much about dinosaurs now that using one as a one-shot monster makes no sense to me. I envision dinosaurs as living in groups, with a tight ecosystem of predator and prey...an ecosystem that dinosaurs are the dominant species of...unlike reality where humans are the dominant species. If I include dinosaurs into a D&D setting then by default they would be the dominant species and they would trump every civilization in my fantasy world. Reality dictates they can not be in a fantasy setting. (Yes, I know reality has little to do with fantasy but when mixing real life with fantasy I prefer to err on the side of reality. To do otherwise can cause the players to question the underlying immersion of the setting.)
Now I could have a Lost Valley with Dinosaurs, but if I'm going to do that I would prefer to use the fantastical. I'll add in flying lizards that shoot lasers from their eyes before I use something as "ordinary" as dinosaurs. I am not against using dinosaurs in rpgs, just not in my D&D.