I have been following this thread for a long time, and although I can't really add any meaningful explanations I wonder if it would be helpful to report my own experience.
I first took MDMA at a few parties around 1998-2000 in San Francisco. It was as amazing and magical as everyone here reports, I can still remember those evenings in vivid detail. I didn't take it often enough to develop a tolerance, in total I probably did it 3-4 times back then. Eventually I moved to the UK, where I took MDMA again at a party in 2017. Over the course of the evening I took 2 pills, and it was absolutely mind-blowing, perhaps better than I remember from San Francisco days (then again, I had never taken such a high dose before).
Since then I've had maybe a dozen more experiences with MDMA in the UK, sometimes at home with my partner and sometimes at a party, and almost always with reagent-tested (but not with lab verified) product, and always at least 3 months apart. The experience has been really nice, and although it's nowhere near the mind-blowing explosions I remember from either San Francisco in the late 90's or from that one party in the UK in 2017, it's still been very good.
I've read and re-read the descriptions everyone has posted about meh-DMA, and I wouldn't say that describes my own experience. I do get some mydriasis, but not full dilation like I used to get. I also become very pro-social and chatty, but not as utterly in love with the universe as I used to get. And I certainly become more tactile and sensual, but previously I could bliss out just touching a couch or the grass, and now when I take the drug I feel really pleasant but I don't go around stroking the furniture. I wanted to post because I feel as though the thread has made a clear division between meh and magic, my experiences have been somewhere in the middle.
I'd also like to say a few things about the science, although I am not a chemist and am wildly outclassed on this thread!
I believe that the early MAPS studies employed a dose of 125mg plus a booster of 62.5mg, for a total MDMA dosage of 187.5mg (I think they use a little less now). MAPS is using a lab-verified pure drug. The MDMA commonly available in the UK has a purity around 80%, so if we want an equivalent dose to MAPS we'd have to use somewhere around 250mg of crystal. Given that the MAPS subjects are MDMA-naive and experienced users might tolerate a little higher dose, something like 250-300mg wouldn't seem to be unreasonable.
I often see recommendations online for dosages around 1.5 mg/kg, but this isn't what MAPS is doing at all (not least because MDMA metabolism doesn't follow linear kinetics). I tend to take around 150-200mg of reagent-tested crystal, but reading this thread and considering the MAPS numbers, I may try increasing that. It would be annoying if the only reason I have been having a meh experience is that I misunderstood the dosage recommendations!
Lastly, if the problem underlying the meh-DMA experience has to do with sloppy synthesis (and that seems to be the factor underlying all of the main theories on this thread), I'm afraid I'm pessimistic that this problem will be solved while MDMA is illegal. Highly accurate, reliable, repeatable pharmacological synthesis requires a professional industrial lab. It requires fastidious scientists, bullet-proof protocols, and accurate, reliable reagents. I am hugely impressed by the work of the people on this thread, and particularly by the determination that
@indigoaura has shown. But even if we could identify synthesis contaminants or stereoisomer variants, would an underground lab be capable of making a clean version of a product whose effects are so exquisitely sensitive to sloppy synthesis?
EDIT: Just in case there are people reading my post and considering the dosage numbers as guides, please be aware that the 80% purity figure is an average number and it's probably not very accurate! I've read lab reports of crystal MDMA purity ranging from 2% all the way to 98%. It's still better to start low and go slow, as with any drug.