While this seems plausible to me -- I'm on Linux, and other tabbed apps like gnome-terminal place the close-window button on the right -- I think that Chrome's

While this seems plausible to me -- I'm on Linux, and other tabbed apps like gnome-terminal place the close-window button on the right -- I think that Chrome's behavior makes the most sense anyway. When you close a tab, tabs shift towards the left. Meaning that the left-most tab in your browser is always the oldest tab, and generally speaking tabs on the left are older than tabs on the right (precluding the possibility that you manually rearranged the tabs). So since tabs on the left tend to be the oldest, they're also the least likely to be closed soon, and tabs on the right tend to be younger, so they're more likely to be closed soon. Hence the close-on-the-right behavior.