If we for one minute ignore DivX with it's dubious legal status, and just concentrate on playing MP3s, MPEG2 (which the XBox 360 can, since it can play DVDs) and Microsofts own WMV/WMA files.
Being able to play these from a standard network share would be very EASY to implement on the Xbox 360. But MS did not. Why? Because they want us to buy their crappy XP Media Center Edition. Having to run a fast PC to decode stuff that could easily be done on the 360 is a ripoff.
You have allways been able to stream MP3 from any pc to your x360.
The fall dashupdate gave you the ability to stream WMV and WMV HD files from any Windows based pc that has Windows Media player 11 (or windows media connect) , both are free of charge to download and use.
This topic, gives you pointers on how to stream virtually any media file you want on your X360, without the use of Windows Media Centre.
I have allready explained to you why they dont allow DIVX etc right out of the box, thats because they would have to pay a royality to whoever owns that codex. The program which this topic is about, is free of charge, and allows you to do just that.