Let's exclude the non-combat and non-reconnaissance troops, the air force and navy for a while.

For line-of-sight-to-threat army troops the special requirement is military discipline.

Historical German army experiences stress that the need for discipline has its roots in the extraordinary demands of combat itself.
The German keyword here is Gefechtsdisziplin - "combat discipline". It's the compound of obedience with thinking and comradeship.
A (small) unit cannot withstand the stress of battle without discipline, thus discipline needs to become natural for army soldiers. (...)
Other than that there are some slightly special requirements (firearms safety, explosives safety, secrecy, psychological stress), which have equivalents in select civilian jobs.
Gefechtsdisziplin has only remote equivalents in civilian jobs, such as some professional divers (doing welding works underwater in teams, for example), some firefighters (I wouldn't add police raid and hostage rescue teams).


note: Combat does not demand that you don't cheat on your wife. It may demand that no ill-controlled long hair creates gaps in your NBC protection, though.