I know of a couple common 210 problems - cracking dashboards, clicking seat rails, window regulators failing, short lived front suspension bushings. The m104 powered variants of course had the customary head gasket issues.
The V6 has the self destructing harmonic balancer issue.
Now the diesel - that's a whole 'nother thing. Like John, I too had so many issues I finally sold mine. It took me five years to learn - guess I'm not as smart as he. Common problems on the diesel include: failing transmission control module (all these are certainly fixed by now), short glowplug life, glowplugs sticking/snapping in the head, leaking o-rings on the fuel lines, mass air sensors, engine mounts. I had all of these problems.
That said, the 210 is an absolutely fabulous car. Just the right size, drives beautifully, comfortable, low NVH, reasonably nice to look at. If it weren't for that dang diesel engine, I'd certainly still have mine...