Automatic transmission died at around 150,000 miles, car wouldn't shift properly from 2nd to D. Replaced with second hand unit at sellers cost
Idle regulator, which meant car would stall at idle, or idle would be erratic. Relatively cheap to fix, but hard to diagnose
Rear electric window mechanism (not fixed, common fault)
Radiator hose (cheap fix, inconvenient in the middle of summer though!)
Sway arm bushes.
Overall I used to not like the 190E, as I didn't think it was a 'real' mercedes, however I was looking for the perfect London car - i.e. relatively small, but big on the inside, classy and nice to drive, but not too expensive if dented and powerful enough for the motorway. I think I have truly found it in the straight-six Mercedes 190E. It is black on grey in colour, and looks quite good when polished. Paint has withstood weather and time very well with no signs of rust (unlike newer Mercedes with paint chipping and rust!). Interior is very solid, basically a smaller version of the 'S Class' of the era, aside from the rear window everything works, which is amazing considering the age of the car. Mercedes has replaced simple, solid, reliable design and build quality with fancy doo-dads in their new cars, most of which owners don't even know how to use.
The gearbox packing up was a surprise, especially as this is the same unit they use in the larger E320/S320 models of the time, and the diff is whining a fair bit, but I have been told this is common for mercs of this age and shouldn't be a problem. The idle problem was a nightmare to fix, as originally though it could be down to dirty/faulty leads/distributor or vacuum hose problem, however was located down to one small valve which cost a few quid to replace. I have never used MB franchised dealers to repair the car, instead I have very well trained independent mechanics which keep the cost very reasonable (less than £300 per year for regular servicing).
The engine seems to be leaking a bit of oil between the block and head, which I have been told spells impending doom for the head/gasket, however will have to watch out. All up this is a very solid, durable car which cruises very smoothly and surprises many at the lights (most think it is the puny 1.8 model, which is way too underpowered for the weight). I wish they still did make cars of this simple, quality design and build, the later model C-Class simply didn't seem to have the quality or reliability of these cars.