I have no experience with other brands but I have 2 Dahon Mu P8 and same thing here...
*slipping seatpost - I had to replace 4 shims and 2 seatposts in order to fix it and I really hope I did ( I have to take the bike for a long ride).
* main hinge - I put 200 miles on my bike, no hinge problems so far.
* creaks and clicks - you are totally right, I gave up and I hope one day they will go away.
*bike falls over - don't have any problem, it feels very stable and solid; in your case it must be the size and weight distribution.
* Flat tire - had only one but the Mu equipped with QR so changing tube is a breeze compare to te Curve D3.

TC

UPDATE: I have tried to fix the slipping seatpost problem but to no avail. I am returning both bikes to the store for a refund. 3 different Dahon dealers couldn't fix that problem so maybe the guys in Dahon California could find what's wrong with the bike. Dahon people said there is only one shim they use but I have received 3 different shims from Dahon Cali and one Dahon dealer in T.O found 2 more from Dahon Speed TR and Dahon Curve D3 - and they are all different !!! WTF.... needless to say that my seatpost still slips and I really have to reconsider purchasing another Dahon. I am so upset,
I have spent hours trying to find a solution for that and when I say hours it's roughly 40 hours !

TC