These, of course, are the core of any D&D-esque RPG, and in keeping with the overall design philosophy of starting with the 1st Edition rules (including Unearthed Arcana) as a jumping-off point, we start with the basic stats: strength, intelligence, wisdom, constitution, and charisma.
There are the several different methods of rolling character scores, similar to what we saw in the 1st edition DMG, and also a section on how to do ability checks. That's a cousin of some of the features of Gygax's mid-1980's RPG Dangerous Journeys, which informed the direction that I took with ADD in several instances.
Comeliness, which was a seventh attribute that was introduced by Gygax in the pages of Dragon magazine and subsequently in Unearthed Arcana, was included in early drafts of the ADD rules, but was removed as the playtest wore on. This was one of the few times I allowed my own feelings on a rule to override what I thought Gygax would probably have done. The rules for comeliness, as written in UA, were game-breaking in their effect (essentially giving an at-will charm power to characters with high enough comeliness scores), and without those rules, there was no reason to break out comeliness from charisma - it would turn into the dump stat I wanted to avoid (a lot of games don't use encounter reaction rolls, either at all or to their full effect).
I know there are a lot of people who do use comeliness despite what I see as its shortcomings, and I fully understand why they do; disagreement should not be construed as disparagement. A new, hopefully more playable, version of comeliness, is in the works (it will be added as an optional appendix in future printings of the Players Manual once it's done and tested).
The stats for attributes go from 3 to 25, but 18 is the normal maximum for most PC races. This is a logical outgrowth of one of the chief goals of the project - to put all of the relevant information on a subject in the same place, without having to flip through ten books to find it all. That is why the information on supernaturally high ability scores was moved from the old Deities and Demigods to the appropriate section of the Players Manual. In keeping with the "1st edition as the foundation" principle, fighters are able to have exceptional strength.
And speaking of dump stats, I came up with what I thought was a pretty neat way to avoid using charisma as a place to dump one's low roll. When selling treasure, the amount that one can get for various types of items is impacted by charisma. So, someone with a higher charisma will get a better price when selling recovered gems, or pieces of art, or jewelry, than someone with a lower score.
Comeliness aside, the section on ability scores was pretty much limited to combining and synthesizing the rules surrounding ability scores that were scattered through several books in the original 1st edition.