I would like to see more negative traits. It makes unit design much more interesting. 

I recently released a mod that put 'weak' back in, but I changed the penalty to -50% attack strength instead of the -2, to try to balance this a little better.  That being said, it's a great trait for reducing production times.

As pointed out above, edit the scout template in unit design, and you'll have the weak trait.  Deselect the other traits, and you'll have 2 slots for other traits.

While I do think that weaknesses were probably getting abused to a degree, this is something that I'm sure could be cured with proper balancing.

But the weakness has two drawbacks, with the only benefit being a lower labor cost.  

The first drawback is whatever the weakness says (e.g. -2 Attack).

The second drawback is that you lose a trait slot.  The weakness takes up one of the trait slots you could have used for something to make your unit more powerful, albeit at a higher labor cost.  So, even though you might be taking a weakness that doesn't impact the unit you're making, you're still making that unit weaker just so you can produce them faster.