Directly from Porsche just before the Panamera was revealed.

Raised wings, higher than the front hood

A tapered front bumper

A sloping roofline (in the 911, this is defined by the rear engine)

Strong shoulders

Organic shape overall

Some of you think the 911 designers are lazy, but you're wrong. Believe me, NOTHING is more difficult than designing a new 911. I've rendered a whole bunch of cars, but getting the 991 to look correct was something that took a VERY long time, and it still doesn't even look perfect. Porsche tends to spend time on the very small mm of design. For example, the 991's door is slightly curves inward (very slightly), as in the Cayenne. The front fenders are wider, the rear end is reminiscent of the Carrera GT etc. As said, Porsche believes the shape of the 911 is already correct, so they just need to change absolutely every small detail. They've been doing this for 47 years, and it's obviously working...

 

BTW, keeping a car looking similar gives it a timeless look. For example, the 993, with a few minor updates, could look brand new. Another good thing about keeping cars timeless is that they never become dated. With Toyotas, you are always going to want the latest Camry. With Porsches, every generation has something special about it, and no new model ever dates the one it successes. For example, the 964, and 993 are similar to the 997, but neither the 964 or 993 look too dated, and the 997 doesn't make them look bad. Do you guys understand what I'm trying to say? Basically, every generation has something the other doesn't make each timeless, and never outdated.