Really depends on the setting. If a setting has a clearly-defined flavor, I'll be leery of anything that throws off that flavor. Especially if it is introduced late in the campaign. Mecha in what I was envisioning as an Arthurian setting? Bubbly 16-year-old girls with pink hair and silly names in Ravenloft? Illithids in Middle-Earth? Characters with my 21st-century liberal's view of democracy and human rights in a Conan game? No thanks.
On the other hand, I've run games in Mystara and Spelljammer, where pretty much anything goes. Some settings need to be restrictive to preserve their feel; others do not.
And the other thing is that certain creatures bring a whole lot of "flavor" baggage with them. By their introduction, they change the tone of a game. Dragonlance races (Kender, Gully Dwarves, and Tinker Gnomes) add a great deal of silliness. Warforged and half-golems add a pulp sci-fi flavor. Athasian halflings add their own flavor (tastes like people), and a "living vampire" or "vampire with a soul" from one accessory or another would add a different flavor (er, again, tastes like people). And, yes, dinosaurs do carry a fair amount of "flavor baggage" in my view.