There are numerous minor wording changes, and a few more important changes.
The biggest is that the Basic Set's "1d6 damage for everything" option is gone.
There are minor changes to a few modifiers.
The attack ranks and fighter option for high-experience demihumans are on the class's XP table, making it easier to implement them.
General Skills are included from Hollow World and the GAZ series; demi-humans taking thief skills as general skills is not.
Some of the GAZ maps are included, along with large scale overviews of the Known World and the Hollow World, along with a few pages of descriptions - a few paragraphs per country.
There is a chapter of optional rules, including allowing demihumans and mystics through level 36.
The encounter rules include some expansions.
IIRC, there are a few extra monsters.
There is a conversion guide to/from AD&D 2E for using AD&D modules.
Note that none of the HW nor GAZ special racial classes are included; only the core three demihuman racial classes (Elf, Dwarf, Halfling) are present, tho' 36 levels are available in the appendix.
Note also that there is a comparable basic set: Denning's big black boxed Basic D&D. It's levels 1-5, and no general skills nor weapon mastery, but otherwise the same as Alston's Cyclopedia.
The Mentzer Immortals box was not included; it was reworked into Wrath of the Immortals, and had some major rules differences; the two are not really compatible with each other.