I view most combats as something out of the ordinary, even in the Deathwatch setting. It's a given, considering the backdrop of 40K, that combat will eventually arise, but I like to make the significant encounters special, and the best way to do that is to make more than a group of foes represented by abstract characteristics, abilities and gear. I try to consider motives and context: in a recent Deathwatch game, the PC group was comprised of "misfits", Deathwatch members from various disparate Kill-Teams that had flubbed, jinxed, or failed to perform a certain task, mission, or simply behaved a bit too headstrong and resistant to squad-action dynamics. This was the background I presented to the Players, the reason they were brought together, and the Players (for their part) performed admirably…the taciturn Salamander Apothecary viewed constant requests for healing of minor injuries (1-7 or 8 points) as a sign of paranoid weakness or fatalism, and would refuse such requests outright, telling his squad mates to "rub some dirt on it and quit whining"; the Blood Angel Assault Marine would hare off after an enemy without assessing any other tactical benefits, such as ranged engagement or scouting ahead to rule out the possibility of ambush, and the Tactical Marines were left either with targets they could not engage at range without risk to their squad mate or needlessly wading into rioting dregs just to pull the Assault Marine from under a poorly driven bus (hordes of rioting and looting civilians, being more a nuisance than a threat, can still drag a Marine down with concerted effort and the rallying cry of a shift supervisor quite unexpectedly); the Dark Angels Librarian's hubris knew little bounds, and the pursuit of personal glory (literal glory hounding) brought him closer to clashing with the "there are no shades of gray when it comes to the witch" Black Templar with each use of his psychic abilities.
Dropped onto a world experiencing the final throes of its fall to Chaos subjugation, tasked with retrieving a relic suit of power armour (Primary) and securing any surviving members of the planetary government (for subsequent punishment and execution; Secondary), most encounters with "opposition" were with rioting mobs of unruly civilians, either loyalists driven mad with fear or turncoats, and these encounters were cinematic at best; engagements were descriptive but lacked dice rolling, as rioters were efficiently leveled with concentrated bolter fire and dispassionate detachment. When it came to significant encounters, each Leader had a particular agenda and a particular method he intended to use to achieve that agenda. Each Leader had a certain area of terrain (a building/structure/ruin/cemetery/etc) they were responsible for taking and securing against loyalists until their overall Warlord arrived. They knew the terrain, they knew where best to deploy their defenses and reserves, they certainly were not stupid or lax or sleeping at the wheel, they knew clearly the reward for failure, and it forced the PC group to consider the benefits of a cohesive front, rather than separatist, vainglorious behaviors.
Combat encounters CANNOT be just a collection of characteristics, Skills, Talents, Traits, Gear, and Wounds. They have to make sense if they are to be a significant challenge. And if approached with this in mind, poor Player/PC decisions will certainly break the spine of even an experienced Kill-Team.