Property-sniffing; what undefined is (and why it smells); why the seemingly-little-known in operator is beneficial and different from typeof/undefined checks; hasOwnProperty; the purpose of delete.
That the Number datatype is really a float; the language-independent difficulties of using floats; avoiding the parseInt octal trap.
Nested function scoping; the necessity of using var in the scope you want to avoid accidental globals; how scopes can be used for closures; the closure loop problem.
How global variables and window properties collide; how global variables and document elements shouldn’t collide but do in IE; the necessity of using var in global scope too to avoid this.
How the function statement acts to ‘hoist’ a definition before code preceding it; the difference between function statements and function expressions; why named function expressions should not be used.