If we are thinking about topics: electricity and mechanics are pretty essential in understanding how most obvious things around us work. But, more importantly, it is getting students to think in a methodical, logical manner. One could argue that all subjects should encourage such a thought process, but with the added bonus of real world problem solving, science offers an essential range of probing, logic demanding activities. That's why people hire scientists! They think logically!
A love and understanding of science starts at a very young age. In fact, the youngest kids are already scientists in how they explore and test the world around them. Kids must be able to question what they experience and learn to seek answers to their questions. This information can come from peers, an experiment, or research through books and other media. Getting the youngest children to continue hone their senses to observe the world around them is the first step to developing good, competent scientists.
Kids have an inherent curiosity, which is the most important characteristic of any scientist, professional or amateur. So at the least, K-12 science should respect and encourage that.
As for other important skills/abilities that K-12 science that should engender:
1. Exposure to all the science disciplines (biology, chemistry, physics, astronomy, etc.)
2. Healthy skepticism in anything presented as incontrovertible fact
3. Ability to sift through and interpret data
4. Importance of control groups in studies
I'll leave there and come back to add a few more after thinking through a bit more.