I have an Optimus G while my wife has a Nexus 4. Different housing, Optimus is more squared and industrial, Nexus is rounded off more. Optimus has LG's custom skin and carrier bloatware on top of ICS, while Nexus has pure Android 4.2. Optimus has LTE, while the Nexus only runs HSPA+. Those are the biggest differences.
There are a few other minor ones like the placement of certain items like the notification LED.
Couldn't say. I usually have a camera around, so I rarely take phone shots.