Dropped shadows make the text harder to read. The toolbar icons have no divisions so reduce efficiency. The scrollbar arrows just look daft. The monitor icon looks like a toaster, as does the harddrive icon.
OS X has completely discarded icons in its toolbar, they're probably not cool enough, instead you have to memorise what different coloured blobs mean.
The iconbar texture is ok.
The icons in XP are terrible. Grey blob instead of a nice simple 2D Speaker icon for the volume control. Blue blob for the IE small icon. Light blue/grey blob for the My Computer small icon, some kind of tiny box for a harddisc icon, and floppy/cd icons have the tiny box and a tiny floppy/cd icon for some reason.