I really like the colours on the back, but they have paired them down with watered down versions on the front and leg. I wish the bold colours had continued over all those areas (and have the bold colours available on iPhone and iPads). As it is, when using an iMac, I would only see the colours I don't like.
Black bezels would make the colours on screen appear bolder. Maybe have white (or "light grey", but that sounds too close to beige, the antithesis of Apple) on some models, but at least have black as others, or have a choice of either. Another option would be to have them in the same colour as the computer.
I would rather have a thicker computer than the chin (thicker wouldn't bother me, especially if it also allowed the PSU to be internal rather than the stupid power brick), but if it is there for the speakers, have speaker fabric, not something solid.