MPEG-4 sounds absolutely wonderful as a standard, but when you actually start trying to implement things based on it you realise that large sections of the specification (like body animation) are aspirational, not having been fully implemented anywhere at all - there is a slightly buggy VRML-EAI-Java-applet using MPEG4 Body Animation Parameters (BAPs) at http://ligwww.epfl.ch/~babski/StandardBody/mpeg4/ , but that hardly counts. The reference player for general 3D (BIFS) is currently pretty iffy too, my former BBC colleagues tell me.

Still, it does seem more likely than not that one day the applications of MPEG4 will live up to its specification, and I look forward to that day. For the time being, it at least provides a unified theoretical framework for the encoding and streaming of many different kinds of multimedia - it's just that applications trying to make use of this framework may not entirely work as intended with anyone else's players.