From a ux point of view, does it make a difference if the close button is on the right of a window's title bar, like on microsoft windows, or on the left side, like on mac OS?
I would definitely go with top right, because more people use windows than mac, and those who use mac, just like you, know that generally close button is in the top right corner.
Remember where are you looking for the close button when you get a layer pop-up in a browser window?
That would depend on whether you are using Windows or OSX and perhaps whether you are left or right handed and whether you are using a mouse or a touch screen.
Nevertheless - a left handed touch screen user on windows will probably out of familiarity with a majority of software expect to find the button on the right.
Generally - don't depart from the fundamental norm. If users expect to find something somewhere (because everyone else on the planet does) then you've got to have a really good reason not to do the same!
Other than that - the typical workflow for a (non Hebrew speaking?) user is from left to right - it would seem more logical therefore when trying to get final closure on a window that it be at the right along with other finalization buttons, notwithstanding the working conditions in my first paragraph.
There are inevitably going to be some for whom the left is preferred and some for whom the right is preferred, so you can only cater for best experience for the majority, not for all.
From my childhood age onwards I worked in Windows only. In windows usually I use to close the window by double clicking the left windows title bar and more of friends use that technique only. Because we always use to read the text from left to right so it is easy for us to close the window quickly if it is in left side. For the Past 5 months only I working on mac now it become more easy for me to close the window just a single click.
Even as a fortunate Mac-only user (since 7 years) I would still instinctively look top right in any context other than my Mac desktop. Facebook has them top right. That's kind of standard nowadays, right?