I haven't read the entire thread, but here is my view on it:
There is an obvious difference between 35mm, MF and LF. How much of this difference is appreciable depends on the enlargement factor of the print.
Comparing these film formats by comparing 8x10 prints will not reveal a huge difference, because a good 35mm can easily resolve 80 lp/mm. Since it only took an 8x enlargement factor to make an 8x10 print with a 35mm negative, there are still 10 lp/mm left for our eyes to marvel about. Considering that young healthy eyes normally peak at 7-9 lp/mm, more resolution will bring no advantage.
With 11x14 prints things change considerably. 35mm needs an enlargement factor of 12x to make such a large print. That leaves 6.5 lp/mm, not bad, but our eye can do better and will notice the change.
A MF negative, on the other hand, only needs a 6x enlargement for 11x14. Unfortunately, most MF lenses peak at 60 lp/mm, but still, that means 10 lp/mm for an 11x14 print are not unusual. This will be a clear advantage for MF over 35mm.
An enlargement factor of only 3x is needed to make the same 11x14 print from a 4x5 negative. Given the relative poor performance of most LF lenses (40 lp/mm), the print resolution will only be slightly better than MF, and to make things even less significant, the difference is beyond most eyes, and therefore, we cannot appreciate a huge quality increase when going from MF to LF, unless of course, we make even larger prints.
For prints 16x20, the print resolution for LF lenses drops to 10 lp/mm (same as 35mm and 8x10), which is very good. MF performance drops to 7 lp/mm (at the threshold of visibility), and 35mm is down to 4.5 lp/mm (even poor eyes can do much better than that).
In conclusion, yes, there is a huge difference between 35mm and 645 negatives, but unless you print larger than 8x10, you might not be able to appreciate it. And unless you print larger than 11x14, LF will not bring you the same performance increase over MF, which you saw when moving from 35mm to MF.
Disclaimer:
There is more to print quality than lens resolution. Increased enlargement factors also have an influence on grain and micro tonality. Print quality will always increase with negative format, regardless of print size, but the above does explain some of the experiences discussed in this thread.