Part of the problem is, is that if you like windows to maximize screen real estate (like I do), then you don't want this to happen.
For starters, the "zoom" button only expands a window vertically, not horizontally (this is especially bad since this behavior is so inconsistant with other progs, which actually attempt to fill the entire screen space)
However, if you resize the window so that it fills up all the horizontal space as well, when you open up new windows, it shifts the new window about 15 pixels to the right - pushing the right side of the window off the screen. This makes using the window impossible, and you HAVE to re-position the window. Every time.
This is true in Safari 1.0. I don't recall if this was still true in the 1.1 that came with Panther 7b28 or not. However, minor little interface things like this have such a major impact...