I know traits don't end up being very important but for those how would like to make their Cardie traits as close to real as possible, these is the required traits in my mind.
I would first go with Cold-Blooded because well they are cold-blooded at least what we would consider anyway. They are much more content with warm, humid temps. Also I believe the biology is similar to a reptilian species. This I think would be a must because it would be this way for all Cardies, it's just there biology and it can't be change.
Secondly, I would say Mental Discipline. There are a lot of little things that point to this throughout Star Trek canon. I'd say half of this is genetic and half cultural, either way Cardies are always shown as being mentally tough. There is a scene in DS9 where Dukat is repeatedly blocking a Vulcan mind-meld and mocking the Vulcan for it. Not my only evidence but i don't feel like going on about it.
All the other recommended traits are good too,but these are really the core Cardassian traits. The rest is really personal or character preference after these two traits.
Just so you all know my knowledge for this stems from watching DS9 at least a dozen times over, as well as reading a few Cardie arc novels.
I don't believe I'm necro-posting, but if it helps anybody in the creation of their Cardassian's physiology or character background, the folks at SFI Academy offer a class in Cardassian Orientation and has published an extensive PDF regarding Cardassia and its people.
I've found it incredibly useful, and absolutely brilliant. It covers the vast expanses of Cardassian history, the Hebetians, Cardassian politics, military etc.
It also covers the personality and career range of the people, different common Cardie philosophies and so on.
There's even a sizable portion of the PDF that explains the function of their physiology, including the manner in which they reproduce. (Personally, I had a little theory that female lay eggs, but apparently they're -almost- just like mammals. :I )
A great asset for anyone wanting to really flesh out their character for some heavy-roleplaying.
Personally, I lurve my Spoonhead and made constant return to this file for reference.
Huge buckets of love and credit to Carol Thompson and the STARFLEET INTERNATIONAL Course Academy for compiling this and making it easily accessable.
More in-depth and helpful than any Wiki page on Cardies.
I really like SF Academy, brilliant people doing brilliant things. Still awaiting the arrival of my Officer's Training Curriculum, though. :/
They've got PDFs like that for Klingons, Andorians, Ferengi, Vulcans and Romulans as well, I believe.