I have been wanting something like this for some time...
Let's see if Apple can push the envelope, or fall short.
It should be versatile, that being the key word.
Versatile enough to be a HTPC. AppleTV is an appliance, not a source, and thus, doesn't cut it.
Versatile enough to be a micro-server, headless, stable, and reliable, and not power hungry and intrusive.
Versatile enough to be a competent desktop computer, and run dual monitors side by side.
How is it so hard to offer Core 2 Duo and Quad processors, DDR3 RAM, a full size 3.5" hard drive, or 2x 2.5" hard drives in the same footprint, optional BluRay optical drive, and nVidia video processing, perhaps with a laptop-style expandable video card, or even a modest size expansion slot for video. On a stationary machine, why not have USB, FireWire, and maybe even eSata. USB2 does not cut it as well as FW, or eSata for large file volume file transfers, and extensive backups.
If they go with MiniDP video, it should come with an adapter to DVI, minimum, included in the package, if not also to HDMI, or some sort of analog video output, like S-video, or something, or some sort of compact VGA port, for use with legacy video equipment.
Desktops can afford to be more versatile, and have capacity for more configurability, and adaptibility, and even slight upgradeability. Value is becoming ABSOLUTELY more important. Selling a computer today needs to be viable for LOTS of uses by different consumers, for a long time. I know they want to go with planned obsoleteness, and sell computers again sooner. But that isn't going to fly in this economic condition. Products have to be great, not just good, to justify expenditure.
Apple is well capable of creating great products, but if it is luke-warm, like the new MacBooks, with some drawbacks to overlook, it will be harder to sell, that includes missing the proper price point, and significant profit-taking is not exactly a value position in a tight economy. More features for less money, and people will be thinking that they are getting something for their dwindling dollars spent.