Since you were so flattering and effusive, I'll continue my response moving the focus from rules to lore.
What makes you craft different lore for your world?
There are probably two reasons. The first is that I'm a creative person and I enjoy lore building and world building. The second is that existing lore is generally incoherent, in that it was created by a bunch of different authors to describe the little niches that they were working on at the time, and for the most part there isn't a really coherent picture of the cosmology of D&D worlds except for the Gygaxian "great wheel" model. That model is fine as far as it goes, but it was never fleshed out by Gygax fully enough to become coherent in published works and the subsequent expansion of it tends to be of the hodge podge multiple author mode that results in no big picture just a lot of little pictures.
There are exceptions. Settings like Planescape, Dark Sun, and Eberron have coherent big pictures that encompass D&D and I admire that. But it turns out that the "big picture" of D&D tends to get very personal for lack of a better word and while the personal spin that Eberron puts on D&D is great and well thought out, it doesn't fit the game I have ran in the past and want to continue to run. This means the burden of lore building is on me.
What makes you not allow/or insist on the presence of certain races, classes, backgrounds?
Part of big picture lore building is asking "Why is everything the way that it is?" or "Where did everything come from?" The meta-answer is that D&D has all sorts of creators who are dumping every idea that they find into the lore of the game resulting in D&D's famous "kitchen sink" feel. It's interesting to me that the D&D cartoons lore building, that the D&D world was a dungeon world where members of many races from around the galaxy or the multi-verse had become imprisoned either intentionally or accidently over the years, resulting in a "Star Wars cantina" "wretched hive of scum and villainy" aesthetic was probably one of the most well thought out explanations for the "kitchen skin" in the game's history. But, I really didn't want a "dungeon world" cosmology with refugees from other worlds and a science fiction theme under the surface (that actually shows up in Greyhawk or the Forgotten Realms to some extent). I wanted a more pure fantasy cosmology with a single world, suitable to human archaic lore from ancient Greece to the Renaissance - the very cultural inspirations common to most of D&D.
And that ultimately meant pairing down the number of sentient races from the kitchen sink, or at least redefining them a bit to explain the diversity. The most noticeable part of the paring down for me is the complete removal of orcs from my game because the evolution of orcs ultimately stopped fitting my conceptions. Originally I think goblins, orcs, hobgoblins, and bugbears all refer to names for orc-kind that are dropped in works like the Hobbit, and were conceptually all part of Tolkien's orc-kind which were themselves morgul work minor demons created by and in the service of his Satanic dark lord Morgoth. But take the orcs out of Tolkien's cosmology and you need a new cosmology for them, and for whatever reason D&D ended up with two - one for goblins and one for orcs, which became sworn enemies and not members of the same race, each with their own culture. But this struck me as redundant. Each evil monstrous humanoid was serving the same purpose and taking up cultural space that could be inhabited by the other. Of the two inventions, I found the caste based race the more compelling and so I kept goblin-kind and dropped orcs.
Conversely there are aspects of my imagined cosmology that aren't really a big part of standard D&D and are rarely detailed - divine intervention, curses, blessings, spirits and animism, wizardly research and magical pollution, sacred sites and shrines, and races that I invented over the years like the Orine and Idreth. I have some of this written down and some of this I want to write down, but the point is in play I'm always having to invent stuff to cover the stuff the published rules are silent on. For example, the rules don't tell you what the HD of a sentient garden or sentient house is, or what happens when an old man on his death bed blesses you for fulfilling his dying request or conversely curses you for bringing him to this state.
What makes you not allow certain combos?
I don't forbid combos per se. What I do do is remove most of the redundancies from the system so that it's harder to stack bonuses unless I intend them to stack. If you look at what really breaks the game it tends to be when you have X different ways to do something, and instead of picking one, the player picks them all and takes advantage of frontloading and unintended stacking. By cutting down on the rules until there is only one way to do something and avoiding easy stacking except where intended as progression, it tends to inherently make all combos fair and balanced.
What makes you use certain books and not use others?
I honestly don't use a lot of books. The reason I don't use a lot of books I that when I look at a book I count the number of pages that I feel I'm going to actually use. For most published supplements this turns out to be a number from 0-20 pages. If the book is 300 pages, this means that the book is offering almost no value. Since I often have to rework everything anyway, I usually just steal the 5 pages of ideas from the book that I think are useful to me and write down my version of them. I really only buy a book if a considerable portion of it is actually useful, or if I am adapting so much of the material from the book that I wouldn't have come up with on my own don't consider it fair not to send some money toward the author. That usually happens if there is more than 20 or so pages I actually care about, which is rare these days.
