The simple things are :

- it usually runs the Media Center Edition version of Windows XP rather than the normal version

- it needs a graphics card with video ins and outs, like an All-in-Wonder

- it needs a good sound card with digital (spdif) out

Other stuff often includes fast hard disks for storing and streaming video content, a spiffy looking box to fit into the living room decor, and a bunch of low-power-when-idle requirements which I expect everyone ignores in the real world.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8