What unique ability does the letter e have?

In printed language, it can indicate a change in the sound of a vowel separated from it by a consonant. Keep in mind this isn't an ability that e has; spoken language came before written language, and writing uses various conventions in order to symbolize the spoken Word. It is a function of the letter e.

But this might not be what you are going for; other vowels do this too, under certain circumstances. Cat becomes Cato.

Also, it is most difficult to work on any composition for long without using it; all too soon you'll find it awfully hard to avoid

• almost unimaginably impractical, in fact.