Darkvision: Half-elves can see in the dark to a range of 60 feet.
Find secret doors: Half-elves have a 4-in-6 chance to find secret doors when actively searching.
Classes: Half-elves may be Fighter/Magic-Users, and may also multi-class as Fighter/Magic-User/Clerics. Half-elves do have restrictions on maximum levels: An Half-elf can reach 6th level as a Fighter (7th with a strength of 17 and 8th with a strength of 18), and can reach 6th level as a Magic-User (7th with an intelligence of 17 and 8th with an intelligence of 18). Half-elves are limited to 4th level as Clerics. Half-elves may also be Thieves (single-class only), but may not pursue other single classes such as Fighter or Magic-User. As a Thief, a Half-Elf can advance with no maximum level limit, and may also take advantage of any XP bonus due to a high Dexterity score.
Those Half-elves who are not player-characters might have abilities and limitations completely different from those of an Half-elf adventurer. The nature of Half-elves and their place in the world is entirely up to the Referee, and he or she might include non-player characters of any class.