The book is full of engaging, interesting, setting info and hundreds of neat self-contained plots (adventure seeds, if you will). The pitfalls, however, are numerous:
1. There are no maps of interior spaces -- be they sewers, catacombs, or buildings. This was probably intentional, as the product is designed to be easily customizable. The downside here is that without these maps, huge swaths of the city become unuseable without GM-provided maps, whether you draw them yourself or cull them from other products. This decreases the product's utility quite a bit.
2. There is no index or detailed table of contents. This is completely unforgiveable in a product of this size and scope. Finding specific information on topic X is next to impossible unless you have already stumbled across it and remembered to bookmark it. The World's Largest City not having an index is a bit like an encyclopedia not being alphabetized. Using it during actual play, as a reference, is next to impossible.
3. No 'in-line' stat blocks for encounters. That is, instead of following typical D&D 3.5 adventure format (including that used in the WLD) and presenting creature or character stats next to encounter descriptions, the book instead contains an appendix of generic NPC stats that you are referred to for all encounters. On the upside, this saves space. On the downside, it further reduces the utility of a product already severely crippled in terms of being useable during actual play.
The bottom line is that, due to some poor presentation choices, the WLC is a great idea mine and source of adventure seeds but utterly useless as a reference text or actual play aid. This is unfortunate, as it could have been a fantastic product. As it stands, it's merely a bit above average.