It's better to think of the Apple TV as a media extender then an HTPC, since that is really all it is. Under the hood is closed off, so expanding the formats is not possible.

I would like to see the new interface with Front Row, this might be a perk reserved for Leopard that hasn't been announced.

As far as the Mini acting as an HTPC, mine has done the job very well, excluding the issues with EyeTV and trying to record HDTV with a software based USB tuner. With Perian, Flip4Mac and the MPEG2 codecs, it has been able to play all the files I want via the Front Row interface. For having network shares, you can simply create a link in the local Movies folder to them, and they appear in the interface with no noticeable difference.