Tailor your encounters to your party, so they get the most fun.
Look at the class/race/powers your players picked. Then give them situations where those powers will give maximum effect. This way, players get the most bang for their limited abilities.
I'll use an example from my game:
The ranger only has one encounter power, Dire Wolverine Strike, which lets him attack all enemies adjacent to him. The last few fights I've ran, there have been few enemies, and even fewer that are flanking up close. So the Ranger isn't getting to use his one encounter power, and I (now) realize that's fairly boring. In the next fight, I plan on sending several minions and skirmishers who'll engage, giving him the opportunity to jump in and clock them all.
So, if someone plays a tiefling, throw the occasional fire monster in there so the Tiefling's fire resistance gets some play. If someone is playing a fighter (this class's abilities hinge on opportunity attacks), have some (dumb) monsters provoke OAs, and let him tear into them. If someone plays a dwarf, hit them with move powers so their racial ability comes into play. And so on.