We went from having set timers in Halo 3, where a power weapon would ALWAYS spawn after a certain amount of time of being picked up at the exact same location.
Then in Reach, it worked the same way except the player who picked up the power weapon had to drop it in order for it to spawn again.
And now in Halo 4, power weapons appear whenever and wherever they feel like. There's no accurate timer, since the power weapon can either spawn close to where you first picked it up or all the way across the map, in the enemy team's position.
Why did the concept of power weapons have to change so much? What was so wrong with the previous itterations of it? So what's next? In Halo 5, would you guys be surprised if power weapons were dropped directly in front of players at a random pace in order to spice things up?