Here's what I commonly see (and mind that I'm no expert)....
1. In the same vein of what Juro has defined as "creep", I've seen folks who let there rod drift slightly forward during the backcast. I've also seen plenty of full forward casts starting before the backcast has completely straightened out behind and loaded the rod.
2. Not letting the rod do the work that it's capable of. This is a generalized statement, but in a nutshell flyrods don't require a heck of a lot of muscle to use them correctly. Overpowering casts (especially the forward cast) usually lead to sloppy results. Relax and let the rod do what it's designed to do. When loaded correctly, they're usually quite easy.
3. Lack of attention to feet. The standing position of a caster affects the result of a cast if the body is not working in unison. Slight adjustment of the feet can result in better overall body alignment and therefore simultaneously retard fatigue and improve the casting stroke.
These are just off the top of my head. I'm sure there's plenty more.