Stress
The success or failure of the restaurant will be on your shoulders. Expect to work day and night to launch the business and make sure things run smoothly.
And odds are, they won't run smoothly.
Successful restaurants are run at a breakneck pace. You might appear relaxed and friendly to your customers, but as soon as you're out of their eyesight, you'll be literally running to get everyone's orders put together as fast as possible. Just because you act like each person's order is the most important one doesn't mean it actually is. Running, shouting, sweating, and swearing are all par for the course in this business—but as soon as a customer can see or hear you, you''ll have to be 100% chill.
Equipment will break. Groceries will show up late or not at all. Critters might claim squatter's rights in your kitchen. Your staff may be lazy, or loud, or they may not get along with one another. Your advertising campaign may be taken the wrong way and discourage people from patronizing your store.
Customers will get up in arms about all kinds of things and you'll have to placate them, no matter how stupid or petty their complaint is. Someone might choke or have an allergic reaction, and you'll have to call an ambulance and deal with the negative press that's sure to follow. And all of this could happen to you within three hours of opening your store.
Yes, things might be awesome and fly (like a G6). But the odds of this happening are slim to none. When you're in the food business, it's always best to prepare for the worst, 'cause it usually comes with bells on.