your eyes get used to the details and you tend to notice flaws after a while. the issue isn't as much detail (pretty -similar- between 720 and 1080) but you tend to see more x264 related compression artifacts on low bit rate files.
you also sacrifice a lot of colour information, something i never noticed until someone pointed it out to me (since i've always claimed not to be a -colours- guy, rather, i'm more of a -details- person).
check 10000 BC in bluray and try to compress it to 4gb, and you'd see that the skies just do not look the same shade of blue. and this is compressing from 17gb down to 4gb (4:1 ratio). Some films also show more artifacts than others, compare IRobot in its original form, to a 15gb version down to a 4gb and a 2gb one. Note the introductory screen, compression is very much evident there.
thing is, for new viewers, the increased 'detail' offsets all other artifacts/compression-related loss; but the more you watch, the more you might notice flaws; of course, your mileage will definitely vary.
eto naman ang opinion ko...libre na nga bakit pa titipirin? ang mura naman ng HDD real estate para tipirin so kung makukuha mo, why not?
for the sake of science, sinong may gustong mag-compare ng 720p TDK (that you have) sa TDK (that I have - 16GB, 12mpbs)?
in my case, it's pretty obvious the softness of 720p encodes compared to 1080p for most titles.