Elves, everybody loves them (or not), everybody uses them. The basic D&D elves are very LotR-y, and being awkward I decided to do something different with them and not to have them as a playable race in my setting.
So elves in my (as yet unnamed setting) are physically typical elves; tall, slender, very long-lived etc. Their hair, eyes, skin etc subtly changes colour depending on the seasons and their surroundings, so over time an elf which lives in the desert will find his/her colouration washing out to bleached/sandy shades, an elf that lives in the mountains will turn pale shades of grey.
An elf which lives in one place for long enough continues to change (as their skin petrifies, turns to sand etc) and eventually is unable to leave the place they live, losing their personality and identity and becoming a nature spirit (nymph, sylph as appropriate).
Most elves consider this a curse and as a result rarely settle in one place but some become tired of life and accept the change.
Elves don't age and can only die through physical violence. They are very selfish and fickle, have no interest in other races for the most part. They will have been the dominant race at one point but have suffered a fall (basically stolen wholesale from the 40k eldar, but who cares right?). Magic items, ruins and other cool stuff came from them. They are super-fertile and able (and being degenerate bastards, willing) to breed with pretty much anything, giving me an excuse to put in some other races such as beastmen (yeah, Warhammer again).
I was intending to create half-elf variations for all the other races, but sitting down to it it seems slightly ridiculous and unnecessary, so if a player requests it I will, otherwise they just exist as occasional NPCs.