I'd also say that the size and speed of the OS is one of the big advantages. You can turn the A1200 in the sig on and any PC that we have and it will cut swathes through the boot time of the PC. Mind you though, installing SquirrelSCSI has slowed things down quite a bit recently.
However (Sorry about this being negative), but one thing that is frustrating is the lack/expensive nature of the hardware. Yeah, all of us suffer from this problem, but it just occurred to me the other day when upgrading the XP machine. Just plug in a few more IDE drives and the odd PCI card. Done. As for the A1200, I also like the wedge design, but both the case size and the provisions on the mainboard make adding extra hardware difficult. I've often wondered whether to add extra external HDD's and 5.25" drives to the A1200, but this means extra power and space requirements, and this goes in contradiction to one of the main reasons for using the A1200 in the first place - No bulky tower or desktop machine. Yet, a tower or desktop machine is the best option for piling additional H/W into.
Heh, just one of life's little dilemmas. :-D
