What should be in focus is the subject of your image: that which you wish to call to attention.
When it comes to people as the subjects, our ingrained behavior is to look at the eyes. This where we look when talking to someone, its what is the focus of our attention. Therefore, when looking at a photo of a person, we notice immediately if the eyes are out of focus...it makes the image look 'not quite right'. Usually you hear such things as the image being 'soft' if the eyes are out of focus. In fact, if only the eyes are in focus, almost no one will complain about your image being 'soft' even though, in fact, it is quite soft.
Having both eyes focused is a creative decision, but it may impact the acceptance of your image. Adjusting the distance or aperture slightly will bring the other eye into focus, as well instructing your subject to move their head, so this is completely controllable. Of course, I suspect your shot is a candid, and you had no control of your subject's head.
You should become familiar with the DOF or Depth of Field calculations, and understand how this impacts your photography. The excellent DOF Master is a great place to start. For example, with your 50mm, wide open at f1.4, at a distance of 1 meter, you have 2cm of focus to work with (assuming APS-C camera). anything more than 2cm out of the plane of focus will be blurry. Play around with the distances and aperture settings to learn the limits of your lenses. For example, by simply stepping back half a meter, you would gain 2.5 more cm of focus distance, and the other eye would easily be in focus.