Besides the obvious of creating designs for companies and people, you can expect to learn a lot about a wide variety of subjects. Your job will be to create visually interesting art on behalf of whoever is paying you. And to do that properly, you have to really get to know who they are and what they do which means you end up learning a lot about a wide variety of different kinds of companies.
I've done a lot of work for tech companies over the years and I got to learn all about how they market their products; what they see for the future of their industry; what the leaders of those companies think is interesting etc.
Unless your job is to produce the same catalog for a company forever, you'll be involved in a lot of different and cool projects. Of course, you'll be involved in many projects that aren't interesting or fun too, but you can always use those opportunities to do other things well (like production work) and to perfect your skills.
Here's a list of other things you can expect:
• To mess things up and cost people money (printing the wrong phone number on 10,000 brochures, for example)
• For people to not like your work
• To worry that you're not as good as you should be
• For people to think you're amazing for doing little things (I was called a "miracle worker" yesterday for quickly turning a logo white)
• To save the day sometimes and to really bomb on others
• To take criticism personally and have it ruin your day
• To be able to play for a living (I've built lego houses, been on beaches, been put up in fancy resorts, flew to different cities for work, been given expense accounts for food and hotels, created work that was shown on stage by thousands of people...just in an effort to make great design)
• For your design training to change who you are–once you become a designer you tend to obsess about things like colors of your house, how to arrange things on a shelf, making cool cards for your loved ones, choosing plants to put in your garden, the color and style of your cars and clothes, etc.
• For your family and friends to ask you to do work for them (a lot)
• To think you have the best job possible
• To be surrounded by smart and interesting people
• To make a big impact for businesses and groups
That's the short list.