I've owned a '98 E300 turbodiesel for about 3.5 years nows. Mine seems to be about the most problematic 210 around - which is not to say it's bad.
I've experienced a failure of the transmission electronics; a problem which locks the transmission in limphome mode. The car is not immobilized, but neither is it very useful. I've seen a pattern of this failure in early diesels, but never in gasoline cars.
I've also had several glow plugs fail. Again, this problem is unique to the diesel. I've also seen a pattern of shorter than normal glow plug life in the 606 diesel engine.
What else? The engine mounts have been replaced to cure a vibration/noise problem. The CD changer died.
Other than that, just routine maintenance - A & B services, brake fluid changes, and engine coolant changes. I'm seeing about 25K-30K mile life on the Michelin MXV4 tires, which is a bit short for a modestly performing tire, but no real issues.
Let me finish by saying despite a few niggles here and there it is a truly amazing car. Incredibly smooth & quiet at speed. Beautiful to drive. Great seats. High quality materials. I'd buy another one in a second.