"Character Comfort – Logographic-based languages can contain a lot of meaning in just few characters. While these characters can look cluttered and confusing to the western eye, they actually allow Japanese speakers to become comfortable with processing a lot of information in short period of time / space (the same goes for Chinese)."
The concept that your native language can impact information processing speed/density is pretty interesting.
Thats why the "Aoccdrnig to rscheearch…" text is still readable ( http://knowyourmeme.com/memes/aoccdrnig-to-rscheearch ).
That’s all stuff that directly impacts design density and so on.
Movable type was invented in Korea 200 years before Gutenberg, and their version was much easier because they just had to plug letters into a square grid. When I was a kid, everyone learned to write on sheets of paper containing a 20x10 grid. Lengths of books and school essays in Korea are still frequently measured by the number of 20x10 sheets they would take up if printed on such a grid.
When your writing system is designed to fill a two-dimensional grid, whitespace is not only unnecessary but also potentially distracting. Chinese and Japanese don't even use spaces between words. (Korean does, at least in modern usage.) Leaving a large amount of space between lines would break up a block that should belong together.
Probably for the same reason, WYSIWYG editors here tend to insert <br> instead of <p> when you press the Enter key, Western semantics be damned. Extra space between paragraphs is implemented using two <br>'s instead of margins on the <p> tag. In other words, people see no need for vertical space that isn't an integer multiple of the height of a 1-em square.
Because each character fills a square block, if you pack characters as close as possible, a paragraph becomes a rectangular array of dense squares, which is tiring to the eyes. Adding extra space between lines makes it much easier to read.
With Latin alphabets, characters have irregular shapes, so you will have enough "holes" between successive lines, which helps the reader follow the current line.