What Digivenger is describing in the above wu is more accurately combat SF. Most military SF does not dwell primarily on the nuts and bolts of the weaponry to drive the story; it is primarily about the men and women (and occasionally robots) engaged in combat, and the effects it has on those people. These stories may be construed as anthropology, psychology, or sociology, which are all considered soft sciences, but except in a handful of cases,* very little military SF revolves around weaponry.
A few examples:
- David Weber's Honorverse may go into excruciating detail about different kinds of starships used by Haven, Manticore, and other star nations, but the stories are mostly about Honor Harrington coming of age as an officer in the Royal Manticoran Navy and as a member of the aristocracy on her adopted home of Grayson and at home on Manticore.
- Gordon R. Dickson's Childe cycle, usually (and incorrectly) referred to as the Dorsai series, is explicitly about human evolution and only incidentally about the wars fought by the Dorsai mercenaries. Because of the cheapness of electronic countermeasures, weapons have become very simple (spring rifles) with more complex weapons such as the dally gun seen in Tactics Of Mistake deprecated for requiring more maintenance than they're worth. The Dorsai, therefore, concentrate on improving their soldiers instead of their weapons.
- Robert A. Heinlein's Starship Troopers, often referred to as the seminal milSF/combat SF novel, is also the best example. Anyone expecting the frequent (and violent) action of Hammer's Slammers will be disappointed; Johnny Rico is shown in combat exactly three times during the book, which is a Bildungsroman rather than a war novel. Much of the rest of the book is, as others have noted, an argument for the cursus honorum.
- David Drake became famous for his stories about Hammer's Slammers, a mercenary armored regiment modeled on the 11th Armored Cavalry Regiment (in which he served as an interrogator during the Vietnam War). These are quite violent stories,** often focusing on individuals rather than larger units, but Drake doesn't let you forget about the effect of the Slammers (and units like them) have on the interstellar civilization in which they function.
So, paradoxically, a subgenre which is rather definitely soft SF gets confused with hard SF. Perhaps this is because people are focusing on the starships, powered armor, and fusion-powered tanks instead of the men and women in them - who are really what the stories are about.
*Usually very bad ones from the 1920s, but not always.
**The original anthology was condemned by Eric S. Raymond as "carnographic conservative SF", which tells me ESR either didn't read much of the anthology or didn't pay attention to what he was reading.