I was doing an impromptu shoot with my daughter and my wife was assisting yesterday evening.
After going through the images to pick ones to keep or toss, I found these two and wondered why there was such a difference. I recalled after looking at the EXIF data for each image what was going on at the time.
I took the 1st image (small town, very little traffic at this time of day) with my 7D MkII and a 17-40mmL at 40mm f/5.6, realized I wanted more bokeh and compression, so I mounted my 70-200mm f/2.8L IS within 4 minutes, took a few steps back, set to 165mm at f/2.8 and had my wife hold a silver reflector to light my daughter on the "shadow" side.
I'm no pro, just a dad with a camera, but the difference between a "snapshot" looking image vs a good looking portrait, even with L-Glass is knowing your gear and realizing the differences each will give.
There's a pretty stark difference between these two images and I hope this serves as a decent example for someone.