At the time, it was controversial because of the undulating stone facade and twisting wrought iron balconies.  It also feature the atrium pictured here.

Also innovative is the underground garage, which was practically unheard-of at the time.

The structure inside and out is a work of art.

I found the roof to be the most interesting.  These "sculptures"  are in fact ventilator pipes, turning the roof a mechanical maze into a park.

Ventilation pipes.

Ventilation pipes.

Ventilation pipes.

Ventilation pipes.

It's too bad that this level of thinking is not used in today utilitarian designs.

Gaudi studying the lines and strength of the parabola.

A mmodel of Parabolic arches used for support

Decorative wrought-iron railings on the balconies