Academics are HARD. I'm not going to sugar coat it, you have to work really hard at WashU. To be fair, some students work harder than others, depending on your major. Pre-med students work the hardest, as well as engineering students. Business school kids have a bad reputation for not working very hard, and their school is commonly referred to as "B-School Preschool." I've never taken a class there (I'm in the Arts and Sciences school), so I wouldn't know.
I'm majoring in Psychology, which I still think is very hard. It's important to study something you're truly interested in, so that the work is less of a pain, and more enjoyable and enlightening.
Students study a lot, but probably spend at least half the allotted study time procrastinating. Students will complain a lot over how much work they have, and will probably waste hours doing so. I HATE that WashU students do this. I don't see the point in complaining...just do your work and get it over with!
Classes are pretty big in general as a freshman and sophomore, but start to get smaller the last two years. Most of my classes last semester, as a junior, had about 20 kids in them. I still took one large lecture class though. I love smaller classes, because teachers really learn your name, and I think it's much more engaging and thus easier to learn. Some kids, though, prefer lectures. It's definitely possible to still learn in a lecture class though, don't let me discourage you.
Professors are generally very good, although there are some that are shockingly awful. But I think that will happen at any school. Some teachers have surprised me by being so great, and others the opposite. Most of my Psychology teachers are awesome, which is probably because WashU is known for having a great Psych department - thus attracting many famous Psych researchers and professors. It's so cool to be taught by a teacher whose work I've studied! Many students in the Psych school also work as Research Assistants, for credit or pay, under a famous Psychologist. I've done this for two semesters, and I think it's a great opportunity that all Psych majors should try to do. I'd be happy to talk further about this, if you're interested!
I think WashU's academic requirements are pretty lenient. There's a cluster system which I really like, but most students surprisingly hate. The cluster system gives you four or five general areas (math, science etc) that need to be completed before graduation, with two-four required classes in each area. Instead of being forced to take standard english and math classes, we're allowed the flexibility of picking something like a dance class for language arts, or studying Greek mythology for natural history. I love how open the options are, which allows you to break out of the routine of your major requirements and explore other interests, which may even become a minor or another major. I think other students dislike it because they don't like how random some of the classes can be that they're forced to take, since many students don't see the point. They want to focus solely on their major. I disagree.
I think WashU really well prepares you for after college, while also focusing on the here-and-now.