Consistency and unity are what distinguishes a particular font. The subtlest changes in proportion, stroke thickness, and finish can alter the personality of a font and make it unique. Whether text or display, it is important to keep in mind how difficult it is to create a font. Not only are there 26 uppercase and 26 lowercase letters to render, but there are also numbers, punctuation, and all the oddball characters such as accented letters from foreign languages to render. And all of that needs to be repeated if you throw in italics and various weights such as light, regular bold, etc. The unique characteristics that make a font stand apart on its own, then, have to be applied consistently to every one of the above-mentioned.
Since recognition is paramount, letterforms are precisely rendered with clear outlines. Any deviations in those outlines from letter to letter stands out like a sore thumb. It is difficult to achieve that kind of consistency necessary in creating a font, since any inconsistency calls attention to itself and distracts the reader from absorbing the message.
Learn more about choosing fonts in a design.