I did IR/Econ MA after a two-year post-undergrad break
1) Much less time in class. My program required 3 classes per semester, each of which met just once per week (for a couple hours at a stretch).
2) More reading. Much, much more reading.
3) People want to be there. Although grad school is the new undergrad, if you go to a good program, your classmates will be people who have thought a great deal about why they're in school again and what they want out of it.
4) It's cool to contribute to class discussions. This is an outgrowth of #3. I didn't have a ton of real seminars in undergrad until senior year (went to a state school), but classes were very much discussion-based in grad school, and students want to have a say and ask questions.
5) You'll probably take it more seriously. My parents paid for my undergrad; I paid for grad school. Plus, after a two-year break from school, I was ready to work harder than I worked in undergrad. Because I was so prepared to buckle down, grad school ended up being surprisingly "easy." It's just a matter of expectations.