The stereotypical view of professional academic writing is that it is grammatically complex, with elaborated structures, and with meaning relations expressed explicitly. In contrast, spoken registers, especially conversation, are believed to have the opposite characteristics. Our goal in the present paper is to challenge these stereotypes, based on results from large-scale corpus investigations. Our findings strongly support the view that academic writing and conversation have dramatically different linguistic characteristics. However, the specific differences are quite surprising. First, we show that academic writing is not structurally "elaborated" (in the traditional sense of this term). In fact, subordinate clauses--especially finite dependent clauses--are much more common in conversation than academic writing. Instead, academic writing is structurally "compressed", with phrasal (non-clausal) modifiers embedded in noun phrases. Additionally, we challenge the stereotype that academic writing is explicit in meaning. Rather, we argue that the "compressed" discourse style of academic writing is much less explicit in meaning than alternative styles employing elaborated structures. These styles are efficient for expert readers, who can quickly extract large amounts of information from relatively short, condensed texts. However, they pose difficulties for novice readers, who must learn to infer unspecified meaning relations among grammatical constituents. (Contains 4 tables and 8 figures.)