Driving position.
This was the killer for me, if I was just doing loads of short sub 1 hour trips I probably wouldn't have got so annoyed with it, but I do 35-45k miles a year and it annoyed me.
The seat doesn't go low enough.
The base doesn't raise up enough under the knees.
The car is bigger and there is actually a bit too much gap between armrest and driver on the door side, and the window line is so high resting your elbow on that is not comfy at all.
Then we have the steering wheel and cluster, it is all wrong. You should be able to get in your seating position and rest your wrists on the wheel at 10 and 2 o'clock, do that in the 212 and you can't see the cluster, you know you are doing somewhere between 20mph and 160mph but what speed exactly is anyones guess.

So you raise the steering wheel up to see the cluster, and then you definitely can't reach the armrest.
I ended up compromising with not being able to see above 50mph but could see the digital speedo, but had to hold the bottom of the wheel to do this and use the armrest.
As I said, for the first hour this is fine, but at the end of the day when you are tired and still have 100 miles to get home in rush hour it was a real ball ache.
The final straw for me was driving back from South of France, I trip I do all the time, have done it in loads of cars from 3 series, 5 series and the 211, and it has always been fine, it takes a good 11-13 hours usually but stop every 3 hours for an espresso and with my tunes playing not an issue.
I very nearly stopped after 7 hours in the 212 and got a hotel, it was that uncomfy.
I sold the car a couple of weeks later.
Having said all that though, I am now doing 18k miles a year max, so I would consider another one, but do think the F11 5 series is a better all round car.
BMW has got softer, Mercedes has got more dynamic, and I now think the BMW is the better mile muncher of the two, which is not something you could have ever said about the 211 vs the E61 previous generations.