iPads and iPhones also have great support for CSS3 transforms (3D included with hardware acceleration), but some of the other "common" CSS3 and HTML5 technologies do bog down on the slower processor. Your average desktop may not have a dedicated video card and a browser with hardware acceleration, which creates a smaller intersection of features that will work with no brains on iPads and (generic) desktops.
The list, of course, goes on, and it will continue to grow with time. As with the well known deficiencies of IE (floating, clearing, zindex bugs, etc) however, the iPad caveats and corner cases are becoming more of a known quantity. As with IE, just takes a bit more planning.
