For me it's the emotional connection and romanticism of the marque. I grew up around old Defenders and dreamed of one day crossing the Serengeti in one. Wearing my safari hat. In my khaki shirt. Sporting my Ghurka satchel. With my pet jaguar riding along in the back seat.
From a practical standpoint, they've always felt just so tight and solid to me. You feel as if you can go over, or through, anything when you are behind the wheel. They also drive pretty well on-road for such a vehicle. The creature comforts are nice to have too at times. And there's plenty of cargo room for my pet jaguar.
I miss my old Defender but my current '02 DII is a better overall vehicle I have to admit. And my wife has a '08 Range Rover which is a beautiful vehicle. But the most adventure that thing has ever seen is when she rode over a curb once at the local Aveda spa. Admittedly, it was a very tight parking spot. The Range Rover overcame the challenge admirably.