Stress
Over time, you'll have unhappy customers. You're the one who's going to have to look someone in the eye and tell them their affairs are not in order, they've invested terribly, they're going to be in trouble with the IRS, they owe a ton on their return, and/or their zipper is open. (That last one is just common courtesy.) Your decisions and observations affect lives deeply—you are the stopgap between financial solvency and ruin at times, and it's a big, fat, hairy responsibility.
You're going to work long hours and, if you're a preparer, you'll get hardly any sleep at all or see your family for a couple months a year. If you sign an official document containing incorrect information or calculations, or if you fail to properly communicate something to a client in writing, you could be sued for malpractice. It's a pretty stressful job in that sense, but it's a vastly different kind of stress than what a breast augmentation surgeon endures during an operation. Yours is a passive stress, a nagging voice that is always whispering in your left ear, to be followed by a balanced whisper on the right.
For stress relief's sake, though, you might want to invest in a high-quality malleable polyurethane foam rubber ball.