I love priority systems, and that's the naked truth. I identify as an altaholic, having had seven out of 10 classes level capped by the end of Wrath of the Lich King and the other three all at varying stages of Northrend progression. While leveling up these characters, I almost always have the intent of either tanking or healing (provided I'm on a hybrid). I tell myself that this class will be the "one," the hybrid class I get to level cap where I'll resist the lure of playing DPS and finally have a queue time shorter than twice my age.
It never works out. I'm addicted to DPS. Just last week, I made the mistake of respeccing my blood death knight tank to unholy DPS. Within a day, he went from 83 to 85 ... and started queuing for heroics as DPS. I have an unhealthy fascination with putting the hurt on bosses, and a lot of my draw to playing any DPS spec is mastering its priority system.
Priority systems are awesome. They're the basic building blocks of proper DPS; there is nothing more pivotal or simple for maximizing your DPS as learning to manage your priority correctly. Some classes, like the 3.3.6 marksman hunter, have such mind-numbing priority systems that you wonder how people can bear playing them. Others, like the Wrath of the Lich King feral druid, had such complicated priority systems that good feral druids were as rare as a holographic Charizard card.
Enhancement is normally considered to fall under the "hard" section of priority systems. With one proc based attack, a DoT that needs to be buffed before casting, two melee strikes, a pyromaniac stick that hates attacking your target, and a single-target shock spell for use when everything else is on cooldown, we have a lot to monitor. Luckily for us, addons exist. If you're struggling with perfecting your priority, here are some of the mods you should look for.