It is the non-specializing approach combined with depth and (presumably) some systematicity. The latter two are where Western books do not match their counterparts.
It is hard for people who have not seen the above-mentioned encyclopaedia to realise how much depth there can be. I thought, therefore, that it would be useful to give a partial explanation of the book.
As to size, it has 715 large (not quite A4) pages with small type. It's so massive it took a panel of seven contributors, three of whom were pros (highest 7-dan, and all the type who write their own stuff).
As to depth, consider one portion of the middle-game section - the one on invasions and erasures. The first section is on types of invasion (six) and spreads over 51 diagrams. The next section is on selecting the invasion point - 95 diagrams. The section on standard invasions and how to reply covers 170 diagrams. Then come 59 diagrams on standard erasures, with finally 104 diagrams on invading and erasing moyos. That's a total of 479 diagrams over 74 pages. That's no record, of course, but these are quality explanation diagrams, not problems and their solutions. Actually, the encyclopaedia has separate sections for life and death, tesuji, etc that relate to this section. On top of that, this middle-game portion has equally extensive treatments of attack and defence, settling weak groups, the value of thickness, and sacrifice stones.
As to balance, overall, the middle-game portion gets 178 pages, compared to just 115 for josekis. I may have lost touch with the English literature but I suspect that, if anything, the ratio might be reversed here, and if you strip out the problem type format which tend to characterise many of our middle-game books (the encyclopaedia has its own problem sections, don't forget), I suspect that josekis get a bigger slice of the pie over here. If so, I suggest that's unbalanced.
As to quality, you will have to trust me on this, but Zhao Zhiyun's reputation is well deserved. Not being a Japanese 9-dan, he certainly knows more than western amateurs. Apart from the fact that the encyclopaedia focuses on proper explanations rather than problems, when you look at the diagrams you tend to recognise your own games. Japanese authors often focus on positions that are interesting because they are unusual and/or diagrams taken from the pro author's games, which are hardly likely to contain the errors that we make. The encyclopaedia, in contrast, is called "Practical" for good reason. For example, you know the situation where you get a two-space group on the third line between two enemy positions. You're never quite sure whether you should be knocking your knees as you anxiously await the attack or patting yourself on the back for having driven a wedge into the enemy's heart. Either way, you are usually even less sure how to play this sort of position. The encyclopaedia tells you. I can't recall seeing that in Japanese or Korean books, let alone western ones.
As to being systematic, you'll have seen a few lists of e.g. typical L shapes, with and without legs and hanes. Useful, aren't they? But where are these lists when you need them, and what about all the other standard shapes that have no name? The advantage of the encyclopaedia is that it gives all the shapes you've read about and many you haven't, and they are all together in one place, as a superb reference tool (and nicely laid out, BTW).
As to being holistic, apart from (of course) fuseki, joseki, middle game, endgame, tesujis, life and death, there are longish entries on famous players - Chinese, Japanese and Korean - and on famous books. What the book reminds me of is the annuals I used to get at Christmas time as a kid. They were such an Aladdin's cave of delights, especially through the dark winter nights. I don't mean that the present book is for kids, though. It's suitable for even dan players. It may be the only go book you ever need! As it covers the basics so well, I even think it's a shame that so much money was spent, with the best of intentions, by Oriental organisations on things like freebies to the WAGC for a handful of people. If they had spent some of that money on having this book translated (not by me, I hasten to add!), far more westerners would have seen the benefit of the money and we would have a much better grounding.
Finally, I'll give just one example of the insights in the book, for a special reason. The reason is that there was a little discussion some time back when I mentioned having seen "sector lines" mentioned in Oriental books independently of Bruce Wilcox's version of the theme. When challenged by Bruce, I was able to find a Japanese source to prove my claim, but I was unable to remember the others. However, looking at this encyclopaedia again to count the diagrams, etc. I was delighted to re-discover another example, this time from China. But there is a subtle difference, I'd forgotten. At least I think, lazily without referring back to the stimulating Instant Go, that I'm right in saying that Bruce had in mind drawing sector (his term, or boundary) lines in the open centre from the tip of one position to the tip of another for the same side, the idea being that you have to be very wary of crossing that line as an invader. The Encyclopaedia, though, does not use the tip of the position but rather the tip of the thickness in the position. I'm inclined to see the value of the pro's experience in that small but useful point.
The book is 围棋实用全典 and the ISBN is 7-8051-732-2. My copy is date 1998, so it may not be in print.
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He has written five volumes in a series on his research on recent games.