Well, there's only so much you can do in that number of pages. If this books gives a good set of basic tools for handling different eras with the d20 Modern rules, then I am content.
And I hope that some other designers take this material and build upon upon it [betcha it ends up in the SRD in short order], because each era we're talking about could easily support a book of its own [and a much bigger book than this].
Historical reference books are easy to come by...and I turn to GURPS supplements for good gamer-oriented historical details.
What we will probably get from d20 Past are some basic rules for covering different progress levels, some crunchy bits to give us some of the flavor of these different periods, and some era/genre notes for three or four of the more popular campaign types.