@John -- I don't think Yineng is trying to use "as" for comparison here. I think he's suggesting the word "as" in the sense of "being." ("As the owner of the house, he decided to add a garage." / "As a piece of literature, this book is worthless." / "As employees, they receive certain benefits.") You don't need to compare anything in order to use "as" like this.

You cannot put "as" in that sentence unless you start again and compare something to something else, the cities, mountains etc somewhere else/in another country/place. The adjectives and adverbs have not been set up to do this. ""A large diverse country its size gives it a wide variety of landscapes, with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east" ""A large diverse country WITH a wide variety of landscapes, deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east" ""A mega-diverse country its size gives it a wide variety of landscapes, with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east, the same AS only two other Countries namely American and China" <-- very wordy sentence to get an AS in there. ""A large diverse country its size gives it a wide variety of landscapes, the same AS Europe with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east" < ---- getting slowly more concise. ""A large diverse country its size the same AS Australia gives it a wide variety of landscapes, with deserts, tropical rainforests and mountain ranges" <---- removing the parts of the country now just comparing the types of landscape. "As a large diverse country its size gives it a wide variety of landscapes, with deserts in the centre, tropical rainforests in the north-east and mountain ranges in the south-east" The author was not aiming for this structure with AS at the beginning because then you need to compare it with something like an economical advantage etc. The Author is simply stating IT is a large country with various landscapes. "As a large diverse country its size gives it a wide variety of landscapes, with deserts, tropical rainforests and mountain ranges affording it a great economical advantage with a vast amount of tourist income"

I can see what you mean, and I agree with you that in this particular case, the sentence is not quite correct. Remember that Wikipedia is written by the public, so the grammar is not always perfect. The author of this sentence was aiming for a structure like this: "Twenty stories high, this building towers over the other buildings in the area." "Made with fresh fruit, these drinks are healthy as well as delicious." In these sentences, it's clear that there's just one subject, and the first part of the sentence is a description of that subject. So one subject has two descriptions. We could instead say "These drinks are made with fresh fruit AND they are healthy as well as delicious." We can also reword these sentences so that they look more like your example: "A twenty-story edifice, the new building towers over others in the area." "A blend of fresh fruits, this drink is healthy as well as delicious." The subject of the sentences hasn't changed ("building," "drink"), but there are now nouns in the opening descriptions, too ("edifice," "blend"). Again, we could reword the sentence to make the connection clearer: "This drink is a blend of fresh fruits AND it is both healthy and delicious." This kind of sentence structure does sort of imply a causal relationship or an explanation (the drink is healthy and delicious BECAUSE it is a blend of fresh fruit), so it would be perfectly appropriate to use "as" at the beginning. But because the causal/explanatory relationship is implied by the structure itself, it's not necessary to say "as." The sentence you quoted does seem wrong to me, though, because instead of "made of fresh fruit, this drink is healthy," it says something more like "made of fresh fruit, this drink's vitamin content is high." The vitamin content is not made of fresh fruit. The country's size is not a megadiverse country. So I agree with you that there are two subjects here, although most readers will assume that "country" is the subject.

it is not two subjects. it's one subject "the country" with more than one feature [or landscape topography/geography/terrain types]