Philj200: I'm always wary of things like 'common knowledge' or 'received wisdom', so I'm basing my punditry on two things. My own experience of playing Martin guitars (over 50 now) of all types and ages, and some fairly extensive research I did when asked to 'sort out' a 70's Martin belonging to a good friend who despaired of the damn thing. The results seem to be that 70's Martins, especially the Dreadnaughts, were noticeably inferior in tone, volume and build quality when compared to Martins from other eras. The literature suggests that some of the main problems included;
- poorly graduated tops, often left far too thick to compensate for the poor quality tonewoods in use at that time
- Innacurate and badly fixed top and back bracing
- Ill-fitting neck joints
- Uneven finish coatings, often applied too thickly, thus affecting tone
- Pickguards that shrink over time, often causing soundboard cracks
- Inaccurate and badly fitted bindings
- Poor quality machine heads (i.e. cheap ones)
- Badly set-up nuts and bridges
- Necks prone to warping, probably because of poor wood selection.

Martin sorted out pretty well all of these problems by the mid-1980s, and my evidence suggests that the company has improved its standards steadily since then, probably in response to makers like Taylor and Takamine, who raised the bar for mid-price guitars from the 1980s on.

Finally, just to gently challenge your rather sweeping first paragraph; I like most of the Martins I've played, and I've owned more of them than I care to think about; I've bought them even when I couldn't really afford them too. I've just never found a good one from the 70's. I don't have a thorn in my paw either.

And your credentials are?
Respectfully,
Tim