We always mention the quality of DVD/HD disc transfers when we use them as test discs (see our Home Cinema and TV Sound Advice sections each issue), and the diversity is incredible, even now - all depends whether the producers have gone back to an original print and mastered from there, or used a much-used print or (as you say) even a low-quality copy.
The trouble is, some old films simply weren't kept in pristine condition, so only copies/old prints exist - not so bad when watched on an old, smaller-screen TV, but rather disastrous on our lovely new big HD flatscreens...many of which don't thrive with old-movie staples of complex lighting, either (want to ruin a film noir? Watch it on a poor LCD...)
Re Region 1/2 differences - quality variation is more typically on the sonic side. Region 1 discs usually only have the (US) English language to include as a soundtrack, whereas Region 2 discs can be packed with every European language going. This means less room on a disc - space that some Region 1 discs use for higher-bitrate audio. It's not always the case, but a superior DTS soundtrack is often included on US releases that we miss out on, for example.
On the R1/R2 picture side, there is more a culture of remastered/special edition DVD releases in the US that don't always make it over the Atlantic - the Criterion Collection springs to mind. I've got editions of everything from Gone with the Wind and Ben Hur to more modern classics on R1 simply because more thought's gone into the presentation.
Still, not much use playing them on my PS3.... Region-coding: pah!