Worker skill needs to be more meaningful. Workers should start off with less skill (maybe even negative skill in some areas!) skills should have more benefit, and it should be harder to max out a workers skills.
New skill levels
The skill levels should range from a negative number (with negative effects when performing a certain task), to 5.
The median skill level should be 0 (meaning the workers are neither good nor bad at the job), with a chance to be slightly higher or lower.
Higher skill levels (say 3-5) should be rare for new workers to start with, and only usually gained by having experienced workers.
Workers should also eventually have passive skills/traits that apply everywhere and not just to thier job.
Things that make veteran workers panic less when 02 gets low, or maybe some workers eat/sleep less than (or more than!) average.