Compared to other medical references available in the market such as American Medical Assoc, Harvard's, Johns Hopkins, Mayo Clinic or British Medical Assoc medical / family health guides, etc. the Merck's Manual is definitely the best and the most complete & informative. With contributions from around 300 medical experts, no other guide comes close to it in terms of quality, readibility and relevance. However, this 1940-page awesome piece of work can yet be further improved. That's why I'm not giving it a full 5-STAR rating.
One feature that can be added to this Medical Manual are a series of FLOW CHARTS with cross-referencing for the readers to "diagnose" their symptoms, complications etc. Doesn't have to be too many - probably around 100 symptom charts would do. These decision diagrams would be quicker and easier to use than the Index. The ones found in British MA Family Doctor Home Advisor and Harvard's Family Health Guide are very good examples.
Secondly, the editors should use the METRIC SYSTEM for measurement units e.g. temperature in Celsius (with Fahrenheit in brackets), weight in kilograms, metres (or centimetres) for height, etc. so that everybody around the world can enjoy reading those knowledge-packed pages without having to convert and carry a calculator every time.
Thirdly, some COLOUR CODINGS (or thumb index) should be helpful for easier referencing since the book contains over 300 chapters. At least, we don't have to refer to the Appendix or Content Page too often. If colour codings are too expensive, shades of grey, light red (as in the diagrams) etc. should still be acceptable.
Fourthly, the middle coloured-pages section of the book (8-page colour inset) showing the various systems of the body should be placed with the flow charts (as suggested above) so that it can give an EXCELLENT OVERVIEW of the body's anatomy, physiology and related diseases and complications.
Lastly, the Index is not that exhaustive. Not all relevant pages are listed e.g. an important page on "thalassemia" (page 994) has been missed out. And entries for common symptoms like "sneezing" cannot be found. May be a REPRINT with CORRECTIONS is advisable.
P/S. A slightly THICKER PAPER would do justice to this book...