I think to Jo Public Audi are precieved as a premium, reliable and well made car which is why the retained value after 3 years for an A3 is around 50-60% where as for a Ford Focus it's only 35% (according to Auro Express at least)
Not having owned AUDIs as long as many on here, I can only pass general opinion on thing sthat are well documented and/or considered in the motoring press and media:
-AUDIS are percieved as reliable and well made
-That they are perceived as a premium brand
-That they are widely considered as making fine interiors
-That they have good resale.
-That they are `desirable`
-That they are (reassuringly) expensive.