Liolel said:
But on the other hand a certain amount of science needs to apply for a player to be able to imagine the world. If in your ice didn't melt when exposed to fire, there is no gravity on a world bigger then earth, living creatures needed breath in solid iron to survive and so on then you've gone to far away from science.
Hmmm....
First off, there is a difference between observable phenomena and science. Science is a method of examining rules from observable phenomena and extrapolating from those rules what other (hitherto unobserved) phenomena there might be. In fact, making extrapolations like this are the only ways in which the initial perceived rule can be determined to be accurate or not.
"Wood burns" is an observation; it is not science. "As the chemical makeup of wood is broken down through heat, more heat energy is released in a phenomenon that we call fire" is science (at least, insofar as I recall).
"Wood burns because, being of composite elements, it is broken down into fire, air (smoke), and earth (ash)" is an alchemical explanation of the same phenomenon. Your world does not have to work using "scientific" principles in order to have the same observable principles (generally) that the real world does.
In the Middle World, diseases are widely known to be caused by disease spirits. No problems there; it changes the flavor, but not the general observable mechanics. It is, however, possible to encounter a stronger disease spirit in the form of a "monster encounter" rather than a Fort save. Also, those who gain immunity to disease as a class ability actually gain this ability because they are protected from the spirits who cause disease.
In Spelljammer, the stars were seldom hot balls of gas unimaginable distances away. From a character's homeworld, the observable phenomenon didn't change (unless good telescopes had been invented), but it sure changed what was possible in a campaign world in a good way. Read the Forgotten Realms cosmology, and you'll note that Spelljammer still has a real effect on that world.
Consistancy to a baseline of observable versimilitude has nothing to do with the causes of said baseline. In a fantasy world, the principles can change, so long as they make sense and the players (and characters) can relate to the world around them in a meaningful way.
Daniel