The MacMini is okay as a HTPC now, notwithstanding its limited capacity HDDs and lack of HDCP on the DVI out.
But, as I said, the ability to swap out drives and GPUs to keep in touch with the latest developments in HT would be a major advantage.
For example, when HD-DVD/Blu-ray drives roll in, change the optical drive.
When GPUs with HDMI output and HDCP roll in (at sensible prices that is), change the GPU.
Aesthetically, the ability to put bigger HDDs and TV tuners inside the computer would also be a big advantage, rather than having all sorts of things hanging off the machine (even if most of the said accessories don't actually look too bad).