Why in the end of this verb adressed to a single person ("you are" - "ty jesteś") there is "ś", but in others there is "sz" ("lubisz", "widzisz")? Is "to be" - "być" the only verb with such a feature?
Yes, It is an exception.
I guess he's asking about the irregularity of the ś ending for jesteś, since most other verbs use sz for that situation (masz, szukasz, kupujesz, bierzesz)
Let me put it this way: is jesteś the only exception?