A critical appreciation essay is a unique academic form, which assumes that its subject is an excellent work, and uses the tools of literary criticism to show ways of arriving at that foreordained conclusion. In general, it is an assignment used primarily within the discourse of New Criticism. Therefore you should address in your essay New Critical themes. Begin with analysis of how Marvell fits the "carpe diem" diem, looking at how such passages as the following exemplify the theme:
But at my back I always hear
Time's winged chariot hurrying near;
And yonder all before us lie
Deserts of vast eternity.
Next, discuss Marvell's choice of iambic tetrameter couplets, the narrative persona, the use of figures of speech, and the syllogistic nature of the poet's argument.