That's a good question.
I don't think there is anything on my new EX490 that I couldn't have on a generic WHS either by substitution using similar applications or by using the same ones. The only minor exception would be the HP MSS 3.0 software. I have seen the HP MSS 2.5 software run on a generic DIY WHS and in a virtual machine. In fact one could probably use a server recover disc from any one of the HP MSS models and use the image on it to create a virtual machine with all the functionality and software intact.
However, with a generic WHS, I think the best way to go is to simple install software that has the same functionality and only when or if you need it.
And I would also suggest building a server with new tech if possible such as Core i3 (with option to upgrade to i5 or i7), USB 3.0, SATA 6G and so on. This way you go a cut above anything that HP has to offer currently with respect to WHS and can easily go beyond whenever they do.
HP EX490 9TB / Core i5 2500k, ASUS P8P67 Pro, 8800GTS, 4GB DDR3, 3x64GB SSD RAID 0 / Core i7 920, ASUS P6T Deluxe, GTX260, 6GB DDR3 / Core i7 860, ASUS P7P55D Pro, 8800GT, 6GB DDR3 / Core 2 Quad Q6600, Gigabyte GA-P35C-DS3R, 8800GTS DDR2 3GB