Ah, the almighty backbend. This pose is tough because it's a total body stretch. I mean, forget the spine flexibility required for a sec, and you'll see we also need ample space in the wrists, shoulders/armpits, and quads. Many of the common mistakes I see in other poses are just easy alignment issues that people can quickly fix. The common mistakes in wheel pose, however, are often due to a lack of flexibility. That is to say, I think many people know how the pose is supposed to look, they just may not have the openness yet. If this is you, don't worry - the flexibility will come with consistent practice, I promise.