Powerlevelling is not the horrible thing some people make it out to be. There are many reasons why you might be interested in this guide. You may be trying to get your travel power, stamina, a new invention set, your first level 50, or perhaps you want to catch up to a friend. Also, in older games like City of Heroes, figuring out ways to maximize efficiency can keep things fresh and exciting.
Make sure your build has good single target damage. Burst damage is especially good, since you generally only have to win one hard fight per mission. A little control doesn't hurt either for locking down extra minions. But really, I think you could do this pretty comfortably with every AT. Then you rush to the end, kill off the minions surrounding the objective, then either kill the boss or click the glowy. Then hit the blue (EXIT) button and collect that big fat mission reward.
This doesn't start to really be efficient until around your teens. Before that, the mission complete reward is kinda crappy. But starting in your teens, it is generally worth about 25-35 kills of xp (it seems to get better and better as you get higher level).
One night, I actually timed my xp earning rate in rushing missions like this compared to street fighting using my wife's ice/thermal corruptor to tag along buffing and healing me out of group. Rushing missions was 63% more xp in the same time, and when I raised my mission difficulty it became 250% more xp in the same span of time.
Once you feel like you can handle yellow minions, orange lieutenants, and red bosses, go ahead and raise your difficulty to at least vicious. This will significantly increase your mission reward. It is my opinion that once you get a nice damage boosting ability like aim, or build up, or something that can lock the boss down, you're totally ready for the red bosses.