MkI compared to MkII:
Less RAM (IIRC MkI's have 8M, MkII 12 and MkIIa 16)
No Ethernet
Slightly less space for drives (height) but most drives are only 12.5mm anyway so that's a minor thing.
No dimming of display.
No telephone mute.
Builtin FM radio (not so great sensitivity) compared to external AM/FM module (at extra cost,
US$99 IIRC). The external radio module for the MkII also has an extra connector for a wired
remote control.
No input for a (future feature) voice recognition microphone.

Those are the things I can think of off the top of my head.

/Michael