Hiya!
What the real question is, is: "How much fantasy is too much before the setting becomes to hard for a human player to recognize and react to"?
Realism (coin size/weight, physics in space, physics on a 1G planet, etc) is important to the point where it gives a foundation for everyone to partake in a shared imaginative story. But realism for the sake of realism...not very important IMNSHO.
If a GM is constantly saying "Well, no, you can jump three times that, because in this world, jumping isn't affected as much by gravity", "Er, actually, yeah, you can jump 30' into the air and land on the ground without hurting your feet because of the whole 'jumping and gravity' thing of the world", "Actually, you can jump out of water as easily as if you were on ground, it's just how it works on this world", "Oh man. I forgot to tell you...fish can all breath air for short periods of time. So yeah, the great white jumps out of the water and onto the deck of the ship and starts attacking. While I'm at it, I might as well tell you that all sharks have human intelligence and can understand all languages. Oh. And they can shape change once a day", "Lastly, sharks are also poisonous and many have psionic abilities, like most sea life....wait, I forgot to mention that too, didn't I?".
If too many "realities" are played with or otherwise tossed aside ...but other "realities" are used as a basis for various rules or requirements, well, you end up with Synnibarr.
^_^
Paul L. Ming