One of the most well-known and integral parts of Dnd games are the combat encounters. Be it a hearty orc slashing or an epic wizard duel, almost every game eventually turns to a physical encounter.
This fact is probably the topic most brought up when describing different game styles, particularly when discussing the differences between male and female Dnd players. It is a common stereotype that male players live for the hack and slash moments of the game and consider everything else just a distraction. And it is true that many PC's use the game to focus on boosting their own power, rather then role-play mushy moments.
But in all honesty, building a good combat can be just as hard as crafting an intriguing plot line. Combat can quickly turn to a boring roster of waiting to make repetitive actions. Therefore it is a DM's task to throw something new into the mix other then the stereo-typical encounter.
My main strategy for spicing up combat involves crafting strange settings that force PC's to think of creative fighting solutions. Instead of a flat battle-field, I'll have the PC's do battle on a descending spiral staircase; or I'll replace the traditional road scene with a slippery ice tunnel. Adding extra elements that make combat harder (such as having to make balance checks or skid across the icy ground), makes a victory far more rewarding then in the traditional scenario.
It's also important to vary the types of encounters. PC's tend to get angry if you continually have combats where a group of monsters sneak up on them...mix those situations in with others:
-Single vs. Many- The whole party gets pitted against a giant
-Large Scale Warfare- Each PC gets a battalion of units to command in a large scale war scenario
-One vs. One- One member of the party challenges a single NPC, a wizard duel, or honorable duel to the death
-Group vs. Horde- The party must combat a huge swarm of rats or entire encampment of kobolds
When designing combats, the really important thing to remember is to play to your characters. Nothing is more frustrating when a wizard chooses tons of area of effect spells, and the DM keeps throwing single opponent fights at the party. Likewise, rogues need combats to use their backstab abilities, so sending them up against battalions of incorporeals isn't going to make you popular.
Taking session turns to appeal to all the different fighting styles of your PC's can be a very rewarding experience for them. If you choose not to do that however, just make sure that every character manages to find a useful niche in the group dynamic that makes them eager for the next round of combat