Elves, in my home brew, are actually descended angels. They gave up their immortality to act as mentors and helpers to the other mortal races, especially humans. Over time, they've lost knowledge of this past and many of their remaining gifts have dimmed. For example, they once had a potential lifespan of roughly 2,000 years (1E grey elves). Now, it's only a few hundred. A lot of this was done to explain some of the extreme ribbon abilities that 1E AD&D gave to elves. A 1,500-2,000 year lifespan is functionally immortal, even for background characters. They couldn't have psionics. They have spirits and reincarnated, rather than going to a final rest in the outer planes. Stuff like that. Note that this tie wasn't common knowledge. Even the players didn't know it until I decided to retire the world and start a new one.
In trying to figure out why elves couldn't have psionics and what that implied, I eventually decided that one of the origins of psionics was a nascent divine spark. This was humanity's great boon and why they tended to be psionic more often than other races. Along these lines, the occasional human could actually attain demigod status. Of course, psionics could have other origins -- scions of those who wielded artifacts often displayed unusual abilities, as did those who were born into magical wastelands. Elves, on the other hand, had a small chance of actually being completely immune to psionic effects (rather than just the 90% resistance to mental effects). This chance was the same as a human's chance of being a psionic wild talent, or whatever you want to call 1E psionics. This represented how fully they'd been separated from their original divinity.
At one point, I added in kender as a half-elf, half-halfling breed. It worked out okay until everyone started playing all halflings like they were kender. So, I had a plague wipe out every last halfling in my setting. Want a short character? Play a (forest) gnome.
In 1E, all undead walked perfectly quiet. I may have pulled that from a Dragon article, rather than the Monster Manual; either way, I liked it. When I was asked why, I decided it was because the undead had an extremely limited levitation effect that let them hover just above the ground. This led to undead who never set off pressure plates or pits, which was a blast. Not so awesome was the PC who ended up being "technically undead" (I honestly don't even remember how or why); he abused it very thoroughly, including walking over calm water (which I should have vetoed, but he caught me at a weak moment).