But the degree to which different letter slant looks inconsistent. /c/ looks more upright, /f/ looks more slanted, etc.
Note that the "vertical" stems aren't supposed to be parallel to each other. Check out any professional typeface, and you'll see they make compensations in this department. For example, /f/ would almost always slant less than other letters, because the curved ends already contribute to the impression of a slanted axis.