The gameboy version adds Mr. Karate from the Art of Fighting series as an unlock-able character.
The now classic music theme for the Psycho Soldier team did not include lyrics on its original arcade incarnation.
As a bonus for every home port (PSX, NeoGeo CD, etc.) not only you can choose via codes to play as the bosses, but you also get a special ending if you choose your team to include Kyo, Iori and the first boss: Kagura. The sequence actually makes sense when you consider the future KOF plotlines, and is not available on the original arcade version.