The linguistic characteristics of text productions depend on various factors, including individual language proficiency as well as the tasks used to elicit the production. To date, little attention has been paid to whether some writing tasks are more suitable than others to represent and differentiate students' proficiency levels. This issue is especially relevant in the context of high-stakes language examinations. In this study, we investigated task effects in English as a foreign language (EFL) Abitur examinations, the high-stakes test qualifying for higher education admission in Germany. Based on texts produced by 362 students, we examined (a) if and how student writings differ in their linguistic complexity and (b) if the EFL Abitur tasks are equally suited to differentiate the students' proficiency levels. We used a broad operationalization of linguistic complexity, including measures from various linguistic domains in the computational analysis of the texts. The results of our mixed-effects models show that student texts differ in their linguistic complexity primarily by the functional needs of the task types employed ("summary," "analysis," and "argumentation"). Furthermore, writing tasks that demand high independence in language performance are best suited to differentiate between proficiency levels, which becomes especially evident in vocabulary choice and usage.