The question of what Doctor Faustus wanted is critical to understanding whether his plan failed or succeeded and, in correlation, whether the price he paid was worth the gain he procured. This, by extension, is critical to understanding Marlowe's central thematic point in Doctor Faustus.
In Act I, Scene i, Faustus is debating with himself over what field of knowledge to make a life practice of:
Settle thy studies, Faustus, and begin
To sound the depth of that thou wilt profess [teach/practice]
One by one, he rejects every field of knowledge from "divinity" (religion) to "physic" (medicine) and every field in between. What he finally decides upon is the
metaphysics of magicians,
And necromantic books [of]
Lines, circles, scenes, letters, and characters;
A sound magician is a mighty god:
Here, Faustus, tire thy brains to gain a deity.
In the entire passage, Faustus reveals that what he wants most--along with "a world of profit, ... of power, ... of omnipotence;" "command" over all that "moves between the quiet poles" of Earth; and "dominion" that exceeds that of "emperors and kings"--is to be "a mighty god" as he uses his "brains to gain a deity" [divinity]. In other words, Faustus wants knowledge and power that exceeds the bounds of humankind’s natural limitations; knowledge and power that rivals the most exalted of the cosmos. The answer to what Faustus wants is compared to what he ultimately receives, true understanding of his story may be gained.