One of the most unique things about Dartmouth is at the same time the best thing about the college and probably the worst. With the "D plan", there are four 10-week long terms a year, and you must be on campus fall, winter, and spring of your freshman and senior years, and the summer after your sophmore year. The rest of the terms you can either take classes on campus, study abroad, do internships, stay on campus but not take classes and work, or do nothing... as long as you complete 12 terms of classes (and of course all your requirements). This is great in that you can get internships in the spring or fall when other students are in school, not be in Hanover in the freezing cold weather in the winters of your sophmore and junior years, and have many opportunities to study abroad. The ten week terms are nice too, since even though they're fast paced courses and finals are upon you almost as soon as midterms end, you get to change courses quickly and even where you live as people are coming on and going off each term. This also gives the chance to make friends with more people than might otherwise happen, since if most of your friends are off campus one term, you're forced to make new ones.
The downside to all this is that your schedule isn't the same as your friends who go to other schools. They go back to college in mid August while you're stuck at home until end of September, and then they're out in early May while you're still at school until June. Also, when you find yourself in a great living situation, great classes, and great social situation one term, it will end shortly and new people, new classes, and new situations will be upon you soon. I would say you have to like almost constant change; I think it's exciting.
Lastly, the very best thing about the D plan and probably Dartmouth in general is sophmore summer. This is the term after your sophmore year where your entire class has to be on campus taking classes. You think, summer school - ick! Oh no, this is AWESOME. It's all of your friends together (since most of them have been off various terms sophmore year), with no annoying upper or lower classmen, and takes on a vibe almost like summer camp. Classes and professors are more relaxed (many students take only 2 classes), much of your day can be spent lying on the green "reading" or hanging out at the dock by the river, and there is even more partying, hanging out with friends, and friendly spirit than usual. New Hampshire is gorgeous in the summer too, with swimming, camping, kayaking, sailing, berry picking, rock climbing and much more all available. Some of my best friendships were made or solidified sophmore summer, and it was definitely my favorite term at Dartmouth.