For those outside Hollywood, you have the power to nix a manuscript or to pass it along to a publisher—but that's about it. You don't even have the final say on publication, so you're really only a middle-man. And middle-men don't have much power. Which is why they're stuck in the middle.
Power in Hollywood is a different story. It depends on how much money you command and on who you know—and sometimes, what you know about them. If you are a big-shot agent with big-shot clients with humongous box-office takes, you'll find you have a lot more influence over how decisions get made than your colleague who doesn't. The truth is small-time agents with lesser-known clients will find they have trouble getting their phone calls answered and their foot in the door at places with bigger money than what they've dealt with in the past. This may sound rude or unfair, and it is definitely hard, but it's just the way it works—unless your dad runs Warner Brothers, of course. Then you'll be the most popular small-time agent in town.
Outside of the moviemaking industry, you may try to exert the same power that you have over your assistant over everyone else, but you are unlikely to meet much success. Your power to make voices tremble and to have coffee arrive with exactly two ounces of cream and a half-pack of Splenda exactly three seconds after you sit down at your desk without prompting your underling assistant into action expires at your office door. Of course, if your ego is ever suffering, you can call and email your assistant at 2am on a Friday night demanding he find you a place that serves truffle sauce on pancakes for your client in Aspen, CO, before you wake up at 6 tomorrow morning or else, and then inform them the next morning without apologizing that no one was in Aspen, CO, to begin with. Yeah, you do have the power to do that, but it isn't recommended since afterward you'll probably have to train someone else to give you coffee with exactly two ounces of cream and a half-packet of Splenda exactly three seconds after you sit down at your desk.