Having taken a quick skim through the other two books...
Again, layout is an issue. Some of the tables could be tweaked for better look with just a few seconds each. Several parts of both books are in 7.5" wide single column - that's hard to read, especially for those with certain visual-based reading disorders (esp. Dyslexia, Scotopic Sensitivity, Macular Degeneration). The GMG's tables for magic items are in small fonts and have overly large cell padding for the font/face/size used.
The GM's Guide general rules section is organized in an unusual manner: Alphabetical by keyword. Makes things easier to find, but puts Gold several pages from Money. (Side note to Paige: cross-reference entries, eg: Coinage - See Money would be useful in several cases.) This is great for reference, not so good for initial learning. There are designer commentaries interspersed.
There are some interesting deviations from d20/3.5 - like Resurrection always fails on Elves... and why... are buried in there.
The monsters in the bestiary are mostly familiar, but some are subtly altered.
There's an entire game world buried behind these, an implied world (as a trope set) that's as different from 3.5 as it is from Moldvay.
Damnit, now I want to delve more into it.