The feeling that the result MATTERS. If it's just another fight, ho hum, we won, we go on, we fight again, then it doesn't matter how great the backdrop, how innovative the terrain, how novel the props, it won't be remembered.
I remember the scene where my rogue killed a prisoner whose presence was going to bring down too many enemies on us - I decided that discretion was the better part of valor, and killed him to save us. We still laugh over the story I spun to convince the rest of the party that an invisible enemy did it.
Other encounters we remember are ones that shook the foundations of the campaign - like when the party revealed that they could use magic (and LOTS of it) in a place where magic was forbidden except to the powers that be. And when the PCs discovered that a hero they had believed in for most of a campaign was an absolute fraud - and the knightly order based on her life was really not very lawful after all.
Note that while combat was a part of some of these scenarios, the thing that made them exciting was the repercussions that came from them, not the combat itself. Like - should we kill the evil Baron and flee, branded as murderers, or let him live and go on living here ourselves as heroes?
That's my take on it, anyway.