Non-vertical tab edges make it easier to see which tab is on top.

Note that only the active tab is curved; the other tabs have nothing but a line between them. Here's a comparison using the latest UX Nightly:

Old: http://cl.ly/image/221j3J1j3R2Q New: http://cl.ly/image/1b1j1m2K2k24

The old one is showing 13 tabs, the new one is showing 12 tabs and the edge of the 13th one. However, the new one is way less cluttered and easy to read IMO.

I'm not sure what you mean by inconsistent, but I imagine the point is to give a slight depth -- better highlight the foreground tab.

The inconsistency is based around a scenario like this.

Open 4 tabs. click tab 1 and then tab 3. What order do you display the tabs in? Does tab 1 overlay tab 2? If closing tabs don't to the last open tab this now means that the visual stack is not accurate to how the program operates.

I understand the depth but there are better ways to show it that don't take up as much space nor have to deal with issues like visually showing the tab's z-order

The Australis inactive tabs don't overlay. Take a look at the mockups. Only the active tab has a background and border styling.

Chrome stacks away from the currently focused tab. If tab 6 of 10 is focused, 6 will be in front of 7, which is in front of 8, and so on; similarly 6 is in front of 5 which is in front 4, and so on. There is a subtle visual lead to the focused tab. If you then select tab 2 for example, tabs 3 to 6 will be restacked for consistency.

I loathe that behaviour, it was one of the things that bugged me about Chrome. If I close a tab, I want the focus to go back to the previously viewed tab, not the previous tab in the z-stack.

You are assuming tabs are a stack. They aren't.