Suspension bushings are worn.
Driver's seat sags (which is very common in older Mercedes-Benz models) and needs to be rebuilt/re-stuffed.
Instrument cluster dimmer rheostat failed (also a relatively common fault in older Mercedes-Benz cars).
Consumes about a quart of oil between oil changes (every 3,000-3,500 miles) which is quite common for these older Mercedes-Benz diesels, and not bad considering I do most of my driving at 70 MPH (higher engine RPMs = more oil consumption).
Reverse lights are non-operational.
Two window regulators broke.
Power antenna is non-operational.
I bought the car from the original owner because it was a Southern California car with minimal rust, and it had a recently rebuilt transmission and a new air conditioning compressor. I had done much research on what kind of second car I would be purchasing as my commuter, and I found this car after looking at quite a few other cars. The W116, W123, and W126 Mercedes-Benz diesels are unstoppable. With valve adjustments, transmission fluid and filter changes, and oil changes done at regular intervals, well over 300,000 miles is perfectly possible, with even 500,000 miles not being a stretch.
I drive the car 100-110 miles each day on California highways as a commuter, and I love the car. I gets a solid 25 MPG combined city and highway mileage, no matter how I drive it. I expect the mileage to improve in the summer, and I don't think 28 MPG is unlikely in the summer with a conservative driving manner.
One word truly describes this car: SOLID.
The build quality on this car still shames Mercedes-Benz cars that are built today. Mercedes-Benz used to use the slogan that their cars were "engineered like no other car in the world." This car, along with almost any other Mercedes-Benz of its era - is proof that that slogan is the truth. My 300D really feels like it could run forever, and I have no doubt that it was purposefully designed that way.
From the slam of the doors, to the cast-iron construction of the diesel engine, to the simple and no-nonsense layout of the interior - the car feels unstoppable, and I love it more and more everyday. It soaks up the miles with a comfortable yet taut ride, and it has enough power (torque) to not feel flat-footed in any situation. Older Mercedes-Benz diesels aren't that slow if driven correctly, and with a general understanding of the torque-band of the engine. I don't even have a turbo model, and I have no problem cruising at 70 MPH on California highways, though it's not super-quiet yet it is tolerable.
The vacuum system can be finicky, as it controls everything from transmission shifting characteristics to the door locks.
When you buy a W123 Mercedes-Benz, your list of things to fix on the car becomes never-ending, because you want the car to shine like it should, and to be as perfect as it can. The small things may break (as with any car that is over 27 years old), but the true essence of the car is made for the long-haul. The car is easy to work on too, and cheap to work on if you do the work yourself. The oil change process is so simple. The diesel engine is a solid, simple, but practically bombproof motor that is fun to drive either slow or fast - the torque is fun to work with, once you know the motor's nature. Diesels rock.
There is a reason that these cars are used as taxis in the Sahara desert - they never die.
I love my W123 300D.