I think this is pretty important too...
Originally Posted by John Carmack
It is a little bit of a shame that, despite the raw horsepower, we are hampered by kind of the arms-length API interface. Because it is... unhappily true that we have the consoles here running at sixty frames per second, and we could have these massively more powerful PC systems that struggle sometimes to hold the framerate because of unnecessary overheads. Where, if we were programming that hardware directly on the metal the same way we do the consoles, it would be significantly more powerful.