With all the multimedia that people are aggregating, and the proliferation of home networking, I'd say that Windows Home Server stands a pretty good chance of taking off. My primary concerns about the market longevity of this product offering include security, Microsoft's product lifecycle expectations for the Home Server, and what hardware requirements will be for upgrades and future versions.
Security: As has already been mentioned, if you make something readily accessible by the average home user from anywhere, you have significant potential for security holes...whether those holes be from the software and its default configuration or from the home user "knowing" what settings should be used. I'm also curious to see how MS will implement Windows/Microsoft Update - specifically will the patches/service packs require any end-user involvement. It's been my experience that when it comes to the average home user, once purchased and installed, computers and peripherals tend to be a lot like that RONCO product... set it and forget it!
Product Lifecycle: This ties into my concerns over security. How long will MS support the product (issuing patches, service packs, and other "enhancements")? What is the upgrade path and schedule?
Upgrades (hardware and software): Once you buy into the Windows Home Server, get it configured, and then dump GBs worth of files on to it, what happens when MS releases a new version? Will you need to replace your existing hardware because the requirements have say....doubled? I'll also be watching to see how many variants show up. I'd hate to see Windows Home Server Basic, Windows Home Server Premium, Windows Home Server Ultimate, Windows Home Server Pan-Ultimate...you get the picture.
Personally, I think I'm more inclined to take an old desktop (loaded up with whatever extra drives I have on hand), and install a Linux distro and configure SAMBA/NFS.
It'll be interesting to see what happens anyways.
