To be evaluative is to consider or judge something carefully. Find yourself deeply contemplating whether the new polka dot paint job really brightened up your kitchen? Then you are looking at those dots with an evaluative eye.
At the core of the word evaluative is value. When you judge something, you are determining its value — like the value of a mathematical equation. Though both value and the related verb evaluate have some place in math classes, you will more often encounter evaluative in situations that call for critical thinking and judgment. So you will probably want to rewrite the boring ending before an editor turns her evaluative pen on it!