So, you want to know what might break or wear out on your Canon Mark II?
The following is my personal experience, of course, your results may be quite different.
I use my Canon Mark II on a daily basis in some of the most punishing environments (apart from combat photography) that you can imagine. Hot, cold, dry, wet, dusty...these Mark II's have been through it all. They have been "around the clock" (after 9,999 frames the counter re-sets) more times than I can remember. I take care of these cameras because they are my livelyhood, but I don't "baby" them. They are my tools and if it would drive me crazy if I had to handle them like expensive, delicate, breakable instruments. These are professional grade tools, made to "take it". In my experience, they can and do "take it".
the course of the last few years, I have only had a few issues (knock wood) to
The paint has worn off (see example here) and I have dropped my Mark II with the 70-200 mm lens off of an elephant, about nine feet straight down onto cobblestones in November of 2006 (see that story here). Amazingly the only damage was a bent lens hood.
The original camera straps have worn out after extended use and I replaced them with the Canon L4 strap (see that story here)
To be continued...