Rust!
I had a 2002 C-class from new, every panel was rusted when I got rid of it last year. The worst bits were: the 4 wheel arches, the bottom of the doors and the tailgate (it was an estate).
Mechanically it was fantastic (it was a diesel), bodily it was a disaster.
If you get one that is 2004 or later it will be galvanised.