So we have a lot of variables when we are talking about fantasy worlds. To avoid the OP having to define too much when asking a question and where it is appropriate I'd like to suggest we create/utilize an "earth-like world" tag. This ensures we are answering the question in the framework of the physical, cultural, etc. rules of Earth.
1$\begingroup$ I like the idea for worlds that are physically similar to Earth, but I'd rather not mix culture into it. We have a lot of cultural variation just on our planet; I'm not sure we can identify an "Earth-like culture" (unless it's that there isn't one :-) ). $\endgroup$– Monica CellioSep 24, 2014 at 22:13
1$\begingroup$ If a tag seems appropriate when posting use it. If we need to clean it up later we can. A tag should not be use as a replacement for content of a question though. $\endgroup$– ChadSep 25, 2014 at 13:20
$\begingroup$ It is not meant as a replacement for content, my first thought on many of the questions is, well is this an earth like world or your own creation. I suppose the alternate is consider it earth-like unless otherwise noted... $\endgroup$– JamesSep 25, 2014 at 13:56
$\begingroup$ What exactly would be Earth-like in such a world? The species? Geography? Distance to the sun? Atmosphere? In imaginary worlds, these don't necessarily stick together. Therefore, it would be too broad and confusing as a tag. $\endgroup$– Mast - on strikeOct 5, 2014 at 13:29
$\begingroup$ The point of the tag would be that unless otherwise specified the person answering should treat any questions they have about how the world works as they would treat questions about planet earth. In the end it can/could help eliminate comments asking questions like this. $\endgroup$– JamesOct 6, 2014 at 4:37
$\begingroup$ We do now have an earth-like tag. $\endgroup$– HDE 226868 ModApr 18, 2015 at 21:00
While the idea is interesting, I strongly doubt we need a tag specifically for that.
If more precision is unnecessary for the purposes of the specific question, the OP can just write "assume an Earth-like setting" or something similar to that within the question body itself; I've done pretty much that myself, such as here and here.
If more precision is needed (which I see as likely being most cases where a statement like that would be used), the relevant details will have to be spelled out anyway.
Hence, I don't see a tag adding much value either way.
Not everything needs to or even should be expressed with tags. Tags can also easily be misunderstood if people are not familiar with the scope of the site, and we should expect people to come to this site and not be intimitely familiar with its scope (or even how the Stack Exchange network works at all). We regularly have people coming to the Amateur Radio SE thinking that just because the site has "radio" in the name means that everything even peripherally related to or simply called "radio" (even when it isn't; consider "Internet radio broadcasting" for example) is on topic, when in fact the scope is fairly well defined following extensive meta discussion.
Our Earth is comprised of a huge variety of settings, everything from the Mariana Trench to Mount Everest, a large number of different biomes from arctic and antarctic to tropical jungle, greatly varying terrain including both huge flatlands and such mountain ranges as the Alps and the Appalachians, and a huge variety of living species spanning the gamut from incredibly generalistic to highly specialized, including tardigrades, cheetahs, humans, elephants and blue whales.
Trying to condense all of that into a single tag seems difficult, unnecessary and possibly even counterproductive to me, and risks causing problems like those somewhat peripherally discussed in Are geographically typed planets realistic?.