How to describe strength of keeping things simple and beautiful

You could say he has a flair for elegance:

a. Refinement, grace, and beauty in movement, appearance, or manners.
b. Tasteful opulence in form, decoration, or presentation

“Straightforward”, “balanced” and “clear” are positive terms to call someone’s work skills that exhibit this aesthetic – also “effective” is a very positive term. (Very “bottom-line” that one.) “Impeccable” is even more forceful. “His design work is impeccable” is, to my mind, about as good as it gets.

Some cross-reference of words you yourself used:

http://thesaurus.com/browse/simple

http://thesaurus.com/browse/beautiful

http://thesaurus.com/browse/exquisite

http://thesaurus.com/browse/user+friendly