80% is a long ways from 100%! I have done some side-by-sides of 14.6mp jpgs at 100% and 90% and I just can't justify continuing to export at 100% - the difference is just not clear to my eye even at a pixel-peeping level (which is pretty close with pictures that huge!)
For most online photos, I have them at 85% (set in my online gallery's settings), as well as for small resized versions that I look at on a day-to-day basis. Occasionally I'll dip down to 80%, but never below that unless it's for something that has to hit a certain size threshold (like for an avatar, but even there, you can usually just strip the exif and other attached data and get it small enough.)
I still dislike TIFFs, which is practically dinosaur-era technology. I prefer relatively modern PNG files if I want lossless files as they will be compressed (which TIFFs rarely are) so will be much smaller. (In fact, PNGs can be smaller than JPGs and look better for things like screenshots of applications on your desktop, etc.)
And heck, some of those NEFs out there have lossy compression, too.
Bob: true about file size vs quality except that compression is a steadily advancing technology - for example, a very high-quality high-bandwidth video compressed to 4 gigs with MPEG2 (which regular DVD movies use) will look worse than the same video compressed to, say, 3 gigs using x264/h264 format. Same with audio, as I'm sure you know... MP3 is getting pretty long in the tooth and there are many better audio compression formats out there, but it has such a foothold and certainly can be pretty darn good that it's unlike to be replaced any time soon - just like JPG.
Other wars: following compositional "rules" (of thirds, etc) vs breaking them
Photoshop vs others (Corel, etc)
ACR vs Bibble, Silkypix, etc
Giant oversized break-your-neck monitor vs multiple smaller monitors

Covering a child's face in some distracting way or leaving it showing (for online use)
If it's OK to save an online gallery to your hard drive
Machine-gun photos and hope for a good one or take time to compose, focus, and work for a good one
Whether or not "go pro" should influence your DSLR buying decision (and exactly what that means, anyway)
Classic legacy lenses vs new lenses only