If everything a circle it takes way too long to differentiate
I think these are just concepts from fans while following the new design that Apple have shown in the preview releases so far. BUT, not all icons will be round in the final version of OSX as far as Preview 3 is concerned.
Finder, Mail, Address Book, Contacts, Calendar, Reminders, Notes, Photo Booth, FaceTime, Messages etc all have square/rectangle shapes, some with a slight rotation to the icon too. They do make use of more solid color or very light gradient shading in the new icons.
The new folder icons within Finder use a very similar blue to shade between the folders (on the same icon) which isn't as crisp or contrasty as the Mavericks folder icon for example. This seems a step backwards, even with their new aesthetics.
I can imagine people with eyesight issues being able to identify icons much easier though as with the new design there is less clutter/fluff/effects and one prominent aspect which is either a bold colour (less shading/gradient) or a big vector style symbol.
It's a shame some of the nice effects have gone though:
Blurry reflection shown of the icon within the dock = gone
Glowing light to show which app is running = now a strong dot
Soft perspective shadow onto the dock = gone
Shaped icons = now round or rectangular with some rotation
Depth, bevels and inner shadows = softer gradient or solid color
Chrome / Metallic effects = gone
Perspective Dock = replaced with flat rectangle.