According to the admissions website, the official percentage of classes taught by professors is 96%. I can think of a few times when classes are taught by grad students
* Foreign language classes where there's a professor who coordinates the course, but has graduate students lead each of the individual sections. This allows you to have a much smaller class (~20 students) where you can actually practice speaking instead of being in a huge lecture with a single professor.
* Writing classes - same as foreign language classes - the small class allows you share your writing and get critiques from a small group
* Dean's Fellowship courses - Grad students are allowed to come up with their own courses and the dean's select a handful each semester that are actually taught. Most of the time, these are in the grad student's area of research or interest
* Intersession classes - I think there's a lot more flexability with Intersession classes, especially some of the non-academic, more "fun"-type classes. I think any graduate student (or even an undergrad) is allowed to propose a class to teach during Intersession. (Intersession is an optional month of classes in January - tuition and housing are already included in your semester's tuition, so there's no extra cost and it's a chance to either take a 1 or 2 credit class in a completely random academic field or take some less academic, more fun classes like Ballroom Dancing, Wine Tasting, Jump Roping etc)
I remember when I applied, my parents were always asking about the percentage of classes taught by professors. So far at Hopkins, I've only had one class (a foreign language class my freshman year) taught by a grad student. But that said, I don't think having courses taught by TA's is a bad thing at all. Especially in the case of foreign language, they're fluent in the language, and they're a lot younger than professors, which, at least in my experience, has made the classes more casual and engaging. It's a lot easier to have a discussion about politics, life at Hopkins, whatever (in my broken French) with my TA who's just a couple years older than me.
Also, at Hopkins and most big universities, for most of the larger classes, you'll break down into smaller groups once a week (called sections or recitation sessions depending on what college you're at) where you go over the material, get questions answered, get extra help etc with a TA.