The stakes.

Combat becomes boring if players know they won't lose, or will be saved by the GM if they do. Quite often, GMs would be very reluctant to allow players to lose, because that would halt the story. My personal solution is to allow the story to be as emergent as possible in general, and keeping battles to the death meaningful by actually having a decent chance of killing the player characters (I try to keep it at around 30%).

Of course, player character death is not the only (and, after a while, not the most efficient) way to build tension. As a random example: I've refereed a one-on-one game where the player character was escorting an NPC. He was a rogue type, and the NPC was the princess type with combat prowess equivalent to D&D's zero level. There was an extremely tense emergent scenario where he was leading her through the fields infested by goblins. Basic to a fault, isn't it? It was supposed to take, like, five minutes play time. But every encounter roll became a terrific thrill when, due to a mark in my notes, I let slip that, although she was very important, I would not intervene to save her in any way. Three goblins showed up; the player character beat them, but was left rather battered. For him, it wasn't hard to hide or run from them when too many had appeared, but with the lady in tow, every silly encounter with 2d4 goblins became an exhilerating game of survival, protection and teamwork. After a couple of fights with simple, boring old goblins where the girl got almost killed (and he got almost killed trying to protect her), and even I didn't know if she would survive, they stopped burning fires at night, which reduced encounter rate; worked hard to teach her to hold a knife to even their odds just a tiny bit; made camouflage to reduce the encounter rate further; lied prone in tall grass as groups of offensively boring (aren't they) old goblins checked their perceptions; and celebrated like crazy when they finally made it to town. It was very fun, absolutely great, and the single line of pre-planning that allowed for hours of edge-of-the-seat play looked like this:

Crossing the fields. 4 + 1d4/2 days on foot. Encounters 3 per day on 1d6=6 boring goblin patrol 2d4 (+1 at night). !!!No plot armour for princess