it wasn't a matter of training for me, i noticed such right off the bat from casual viewing, both of the old analog CRT sets with their various problems [coarse scan line structure or imperfect interlacing in progressively scanned sets, crawling dots, overscan, rolling, noise, diagonal aliasing, blooming, moire, poor color balance, ghosting, tearing, jitter, judder, flap-poling, vignetting et al] as well as on the newer digital sets [poor upconversion of SD, mosquito noise, block noise, judder, ghosting and/or intermittent jitter/coarse stepping on judder reduction algorithms, patchy color gradients or banding, blocking in action scenes, partial or global suspension of motion/drop-outs, muting of audio, stuttering, poor shadow detail/excessive squelching of low level luminance information, general picture break-up due to low signal conditions, et al]. i noticed these things not because i was looking for them but because they fairly slapped me in the face with their obnoxiousness.
also, i encounter much in the way of imperfections in my audio restoration work, which has taught me to be very sensitive to aural flaws in reproduced sounds. for example, i find almost all analog phonographic media [LPs, 45s and 78s] to be unacceptably noisy when played at normal volume levels, or veiled of low-level details when played quietly. other people who claim that their well-kept records are noiseless, i beg to differ when i hear them, as the rumble/groove roar is to my ears prominent. also, too many records have off-center pressings which introduce wow and flutter, which bothers me more than anything. watching the supposedly remastered "Elmer Gantry" on DVD, i also noticed that the intro music was horribly irregular in pitch, it fairly warbled on the brass fanfare and sounded horrible to me. they shoulda fixed that before they released the DVD. also, reissues of old music/movies which were originally stereophonic when released back in the day, but are now disappointingly in mono on the re-release- i mean, what's up with that?!
too many things to complain about, so i better stop now.