Security/Reviews/Firefox4/XHR.mozResponseArrayBuffer Security Review
Access raw bytes from the response instead of the string version... for binary downloads (ucs2 string)
No endian-ness (worlds is little endian, tough if its not for you).. Could bite you if you're on big endian box and downloading little endian data then feeding it into say webGL. But anyone on a big endian box should be used to dealing with that by now.
Data view for byte arrays can convert data types on the fly if specified (but data views are not yet implemented), but you cannot modify the underlying type.