LootHunter: No, they aren't. The gravity is the same, the weather is the same, technologies are the same. The general laws of nature are the same. Fantasy is a genre that is specifically set in the worlds that operate by different rules. Sure, they can look like our world (DC and Marvel do, even if they have superheroes that defy the usual laws of physics and thus are considered "superhero fantasy") but they always have fantastical elements that don't exist in everyday life.
Cavalary: The thing is that fantasy very rarely concerns itself with the basic data of the planet it's set on, even when completely fantastical it tends to be assumed that gravity's the same, day and year are the same or at least similar enough to make no difference, atmospheric composition, planet type, star type...
To be fair, my mention of gravity was less about fantasy and more about the "being completely set on a different planet" phrase in the OP.
However, I would like to point out that in Equestria sun magically goes around the planet, not the planet around the sun (so the system is geocentric, not heliocentric), while its weather is artificially controlled by pegasi. Discworld goes even further as it's not a planet at all but a giant disc lying on elephants and a turtle. Narnia, if I recall correctly, isn't in space at all - the entire world is limited by the skydome.
Cavalary: some "Earth but not Earth" fantasy can be, either specifically or by assumption, set on an alternate universe Earth, so some laws of nature differ but the actual planet's the same.
Yes. There are many such examples. Sliders is a sci-fi show, but they had an episode with a world where magic exists, and that was explained by different laws of nature there. There are settings like the one in Might and Magic or Cthulhu Mythos, where implied that a sufficiently advanced mind can simply warp reality, thus creating an impression of magic. Or "magic" is achieved by technological means, because again, "sufficiently advanced technology is indistinguishable from magic".
Still, when saying "fantasy world" people usually imply that the world is drastically different from ours on a fundamental level. Not just having different history (that's just alternate history) or no familiar brands (that's just copyright). There should be something different in the very way how reality itself works. And yes, sometimes it's explained by scientific ideas - in that case, the setting becomes sci-fi. And if differences in the laws of nature aren't explained - that's fantasy.
