The Canon 50mm 1.4 is just soft wide open and doesn't fully sharpen up until 2.8. The problem is even more acute on a crop-sensor body. An easy way to see this is as follows:
Put your camera & 50mm on a tripod in AV mode at f/1.4 and point it at something with a lot of fine detail (I used a semi-transparent curtain). Go to live-view and x10 and manual focus as well you can. Hold down DOF preview and watch what happens as you stop down to 2.8. On my copy, I see the image get steadily sharper until about f/2.8. After I did this, I mostly stopped using my 50mm except in extreme low-light situations.
I've been waiting for years for Canon to update the 50mm 1.4. I finally gave up and bought the Sigma Art.