Generation of Ability Scores
Gygax famously argues that 3d6-in-order is a bad way to generate AD&D characters, saying that truly average ("marginal" is the term he uses) characters will tend to have shorter life expectancies and that players won't want to play the classes and races that are the best fits for their rolls. Instead, he outlines four methods for producing ability scores above what would be average if 3d6-in-order was used.
Gygax writes that high-level NPCs should have their ability scores determined by DM fiat, which makes a lot of sense to me. Average Joe NPCs get 3d6-in-order, except that rolls of 1 are changed to 3 and rolls of 6 changed to 4 in order to keep them very average. Special characters, like henchmen (I'm not sure what else, as Gygax doesn't say) get 3d6-in-order, except for any abilities that are germane to their occupation, where a PC method can be used or add 1 to each die that comes up less than 6. This all seems reasonable to me, considering the assumptions about AD&D in the PC generation advice.
The Effect of Wishes on Character Ability Scores
Apparently it was common for players to use wishes to raise their characters' ability scores in Gygax's experience, and wishes were much more readily had than I've ever seen as well. Gygax suggests allowing one wish spell to raise a score by one until the score reaches 16, at which point a wish will raise a score by .1. Personally, I can't imagine being a player with nothing better to spend ten wishes on than raising one of my ability scores from 16 to 17, but apparently that happened. I find it interesting that Gygax pushes the idea that PCs must have above-average scores just a few paragraphs earlier, but here treats the idea of "many characters… eventually running around with several 18s (or even higher!)" as something to guard against.
Characteristics for Player Characters
One hundred pages into the DMG, there are tables for the traits of NPCs. Mostly, these are personality traits, like alignment, intro/extraversion, attitudes towards money, honesty, etc., though physical traits like age, height and weight are also included. Gygax warns against rolling on these tables for PCs for anything other than height and weight, insisting that the players must be allowed to decide on the personalities and choices of their characters. He does allow for players to request rolls on these tables if the DM thinks that the player can roleplay a randomly generated characteristic well enough.
Player Character Non-Professional Skills
As an option, Gygax includes a D% table of secondary skills that PCs may be familiar with, such as farming, mining, masonry and sailing. He leaves the adjudication of these skills to the common sense of the DM, without any mechanical way to determine what a secondary skill allows a PC to do. This is a prime example of where Gygax's earlier admonitions against unauthorized products clashes with my experience. Dragon Tree Press' Monstrous Civilizations of Delos includes a much more extensive set of tables for determining secondary skills and apprenticeship/educational background, with concrete mechanical benefits for each entry, and both my players and myself have really enjoyed using it.
Starting Level of Experience for Player Characters
This section is mostly concerned with how to treat new players. Gygax argues that players who are new to roleplaying should ideally be segregated from experienced roleplayers and allowed to learn the game for themselves instead of being taught by other players. I think this is usually impractical, but a good idea when possible. Gygax is hesitant to allow players to start with characters above Level 1, arguing that they will enjoy their levels more if they've earned them, but makes exceptions for players entering an existing campaign where the other players' characters are leveled and for new players who don't see level-gain as a worthy goal; the idea is that once they've experienced a high-level character, they will want to level their characters up to such a high level.