Has anyone cracked the career code yet? I know it's still early, but how feasible are customizable careers in this game? It's bound to be difficult with all the tasks and such, but modders have been wizards in the past, if it's possible to overcome an obstacle I have confidence that they would find a way.

Off the top of my head, I think the hierarchy in the XML files goes:
career (Tech Guru)
career level (Tech Guru Level 1)
aspiration (Tech Guru Level 1 Objectives)
objective (Play Vidya Gaymz)

I changed objectives: http://modthesims.info/showthread.php?t=536890


The DATA files are a wrench in the works, though.

Corresponding instance DATA files contain the text table references.

What about traits? Are those any more moddable than they were in the Sims 3? No one seemed really to be able to do anything with those. I'd really like an "emotionless" trait for someone who isn't affected by emotions either positive or negative.

Take a look at the EAxian documentation? That contains an example trait and should give you a pretty clear idea of what can and can't be done.

The CC documentation that EA released included the making of a custom trait as an example but no one has managed to do anything with that yet. But it's implied to be possible.

Both traits and careers are easily moddable (in the sense that you could do it right now) but there are a lot of files to fiddle with. Just find the files related to a career and make copies with the new information, and make sure to also provide the corresponding DATA files. I have been working on a tool to create things automatically, but I am not very quick in that regard, so if you want these things now just sit down and gather the files needed (use s4pe and the tools created by velocitygrass).

The careers are divided up into these files (and possibly more):
career: references starting track
career track: references career levels and branches (other career tracks)
career level: references aspiration (general name of a group of goals)
aspiration: references goals

most information in DATA files just mimic the information in the xml files (use the data editor by velocitygrass)
and create string tables using the string table tool available here on mts.

As I said, there is nothing more needed to mod these things, it can be done right now.

(the reason I haven't made such a mod is because I want to focus on creating tools in the first hand)