An interesting idea, for every subject you really only need to know two things. An example:
The Two Things about Database Administration:
1. Backups are life. (Comment: You can screw up anything you want, miss deadlines on projects, or whatever and it likely won't get you fired, but if you don't have backups in a crisis or an audit you're done.)
2. Premature optimization is the root of all evil. (From Knuth, via ESR, and it applies to database admninistration as much as Unix Programming.)
And it seems to be true.