I think that different shapes are important, what bothers me more is the overuse of colour; I'd much rather the icons were duotone, or at least mostly duotone with a focus on looking distinct. I currently have Creative Suite 6 and I don't like the icons for these, as the colour is the main differentiating factor, as the use of letters isn't iconic enough (you have to read them).
So yeah, my biggest feature wish for Yosemite is a second icon definition in app packages that specifies a duotone icon so we can switch.
But yeah, I think having the differing shapes is a good thing, provided we can maintain consistency. This is where we have the big problem; app developers will come along and not know what the rules are because Apple doesn't seem to want to say. Really what Apple needs to do is provide an icon builder tool that makes it easy to create your own round or square icon, and give clear descriptions of what each is intended for.