* Laziness: Does something hold onto the head of a list or does it drop it. Memory characteristics of the same. Unexpected memory usage.
* Compilation: When and in what way is clojure compiled. How to do AOT. How to produce named Java classes. How to create executable JARs.
* Java interop: Given a java snippet, what is the closest idiomatic clojure. What library is used to do foo.
* Performance: How can a snippet of code be made to run in less time. Why should or shouldn't a snippet of clojure be faster. What is the typical speed relationship between a piece of Java code and a piece of clojure code. How to type-hint a snippet of code. What can/cannot be type-hinted. What are and how best to use transients.
I feel like those topics account for a large percent of the traffic on the list. I'm certainly leaving things out. All of the above have been dealt with at one time or another on the list.