When I open a PDF in Preview.app, the green button should have the same behavior as Safari does when pressing this button. Currently, you press it and it goes to a standard size. You have to drag the corner to make it big enough (usually I zoom into documents to make the text bigger, Zoom doesn't account for the wider document).

Even weirder behaviour when you Zoom a document which has 'Automatically Resize' turned on; Sometimes, every time I press Zoom, it makes the window a few pixels thinner. Eventually it gets very small then wraps back to being 'big'.
Oh, and definitely there should be a default shortcut for Zoom. I use it all the time (wishing I could use it in iTunes properly)...

What else... Ah yes, if you click Zoom in the Finder in column view, with only one visible column (eg open your home directory), then press Zoom........ Oops, now column view is annoyingly hard to use, because there is only room for one column. There should be at least 2 columns width here.