Features that I don't use but I think are cool (many people seem to be forgetting about this part of the question) are
Video editing - (iDVD, iMovie) In an ideal world I'd have enough time, energy and attention to get into video, but in this world at this time I just don't have the opportunity to do video.
PCMCIA slot - it's good to know it's there in case I ever need it, but so far I've never needed it.
dvi output - ditto
iWeb - it'd be cool to make webpages easily with iWeb, but I'm not willing to buy .Mac service because I don't think it's a good deal. (p.s. can you use iWeb and just have it export the html and media files in the proper directory structure so I can just upload it to my free webspace?)
.Mac - cool features, but not worth the money.
Photo Booth - would be fun, but I've got no video cam.
GarageBand - I used to do a lot of music composition, but again, I've got no time and energy now.
Fast User Switching - although I've got several users created for myself, I only really use one, so I turned off Fast User Switching to save resources, but now I never get to see the cool cube spin animation!
FileVault - it'd be nice to know that all my data is safe against prying eyes, but I've really no need for it, so it's not worth risking the extra problems I'd have in recovering from a disk problem if I ever have one.
Bluetooth - wireless connections to mouse, keyboard, phone, etc. would be cool, but I'm on a laptop and I've got an old phone so I've got no use for it.
Time Machine - I hope to employ this soon, but as of today I haven't set it up.
Folder Actions - could be super useful, but I haven't had time to set any up.
Smart Folders - ditto
That's about all I can think of for now.