Another list to add to the already decent contributions above for the w204
Satnav volume, if you want to turn it up or down you can only do it whilst its speaking, took far to long to figure that one out! tbf it see's little usage as like most inbuilt systems my phone does a far better job.
The footbrake, in anything other than warm weather is slow to disengage and have to manually pull it up, no amount of getting it cleaned/oiled has sorted that. Also not sure how it passes an MOT as it seeming does very little?!
Still not figured out the voice controls or what it can do, irc its limited to the satnav and with the above point is probably why it stays unused.
The GTO, the worst thing i can come up with is the boot drain holes seemingly get blocked at the slightest whiff of grit, age related issues aside what it does do it does well.
edit that, the rear lights, probably related to the drain holes, but they do like to turn into little fish tanks and rust the bulb holders that can give some interesting lighting if you dont keep on top of it. drilled some holes in the bottom of them that seems to have sorted it but its a common issue for sure.