I'd love to know too, just in general. As i say, i go by David Nutt - there's no real evidence i know of (haven't looked in a while) of long term harm, just some evidence of slightly affected short term memory with heavy users (like cannabis). Notice in the chart Nutt puts mdma quite a way below cannabis (though probably mainly due to the smoking of it, rather than based on THC).
Mdma pharmacologically is mostly like amphetamine in profile (which is also not all that physically bad for you in single controlled 'clinical' doses, hence approved use in adhd) - the methylene dioxy part (the md) makes the other end of the molecule slightly resemble a tryptamine (like psilocybin or serotonin), and so gives a slight serotonin agonism (it takes the place of serotonin at certain receptors (HT2x) - the same way other 'trip' tryptamines work), alongside the amphetamine's dopamine/serotonin release. It also releases oxytocin, thought to be linked to the empathy effect, though the mechanism for this wasn't fully understood (last time i was looking). Sorry, i'm probably telling you things you already know, but neuropharmacology is one of my amatuer interests so couldn't help meself.