It's very normal to compare and contrast a potential new partner to our ex. It's how our brains are wired.
For example, if we own a Great Dane, every time we look at other dogs, we will compare them to ours. If we're very happy with our dog, we would tend to look fondly on other Danes and possibly other large breeds. On the other hand, if our Great Dane were a problem dog, we might see a tiny, teacup dog and wistfully imagine how nice it would be to have a small dog.
You can't stop this comparison from happening - just be aware of what your brain is doing.
Understand that just because your marriage has failed or is failing doesn't mean that your STBX had zero redeeming qualities. You should avoid the temptation to actively look for someone who is the polar opposite. It's OK to notice, but don't assume that just because they seem to be an opposite of your STBX that a relationship with them would be perfect.
Finally, take a look at Kimberley's list above. I'm sure those items aren't the only thing the two men don't have in common. I'm also sure that if her perception were changed, she could find just as many things, if not more, that they have in common. Again, it's just the way our brains are wired.