I believe the interior styling of my S4 is ergonomically perfect. Audi used high-quality materials, excellent design, and is very aesthetically appealing.
The V8 is deceptively quick. The car is quiet because it is well insulated. The best gears are 3rd and 4th in the 3000-5000 RPM band. The AWD system tracks straight and true even in the most torrential of downpours.
The car does not roll or lean and always feels solidly planted. Because of the AWD system there is no torque steer like on FWD cars. This gives me greater confidence when taking on/off-ramps at speed.
The car has numerous airbags available, stability control, traction control, and excellent brakes. The car feels solid and predictable in just about any driving condition.
The quality of materials is exceptional. I paid $28,300 for my 2005 S4 with 25,000 miles in July 2008. The fact that it was a $50k car new meant that it had better build quality, materials, and design than new cars in the $25-30k range.