It would be interesting to find out what the average diameter is for modern day coins. Anything too large is now handled via paper currency (or plastic), so I do not think the variance is anything to be too concerned about. Though if I knew the average diameter, I would probably pick a coin that was very close to that size. Good point on your part!
The biggest factor for me, even to the point of overriding the variance size concerns you raised, is that the coin is the only item on that list that can be used virtually anywhere without looking too out of place.
Regarding CDs/DVDs/BDs, I think there is still the issue of potentially being too big, and thus distracting from the small object that should be the main focus of the photograph. And of course the location issue. The size can be partially mitigated by using a Mini-CD, but not the location.
Hm. This is a silly poll, and yet, we're taking it seriously. It's that coders instinct to want to solve the problem I guess :)
Lol!! I've been doing a lot of 3D graphics work lately (as a hobby), so I think I'm in "how would I create a scene as described in the title?" mode. It's almost like a render challenge. (^_^)
Update: As a side note, if more than one coin can be used then I think that also pretty much negates the size variance issue.
Elda Taluta; Sarks Sark; Ark Arks