Because some people just like a challenge.
Arth 101-102
Unless you really like memorization, this class is quite hellish. The lectures are great, but the memorization -- five facts for each slide -- is a lot of work. Also, since virtually no one has written an art history paper before (at least no one who is taking 101-102) you just write the papers like regular papers, and it turns out that art history papers are supposed to be totally different, and since there are only two papers in the semester, it's hard to improve much.
Estimated weekly workload: 2-5 hours
Math 346
Topology is hard. And the final exam is really hard, definitely nothing like what is in class. Since the textbook (Munkres -- pretty much the only textbook in existence) is created for a two-semester graduate school course, and this class covers it in one semester as a 300-level course, you have to skip a lot, so it's hard to know what to review in the textbook. Also, there are very few examples, and a lot of long proofs, and almost no illustrations.
Estimated weekly workload: 5-10 hours
Greek 101-102
It's not the work itself that's hard, it's the sheer amount of it for a 101 language course. You have to learn enough grammar to read Xenophon and Euripides by spring (which is, by the way, totally worth it).