Well I like this question, because often the focus seems to be the differences, and you ask about similarities
Studying for the exam, there is a world of differences between them, they are just sooooo different!!! And we talked about pro/con for each of them.
When it comes to the practical approach, it is actually only the façade, if you go deep enough they are really not that different (e.g. from a philosophy perspective)!
If you want similarities of what they do not take into account, the list is long...
At the surface, you might look at these matrices in the other link/thread (e.g. see the link from Sathis), and find similarities like, okay they all focus on the importance of the management responsibility or the process/ongoing concept etc...
That said you actually answered it yourself, they all focus on their own approach! - but can they be integrated with other approaches!?
With inspiration from Revees & Bednar (1994)'s multiple approach, I for some years ago suggested some alternative integrated views, here u get two examples (the focus was only the product):
Time/cycles: A product goes through all the "quality approaches" depending on its "age"/situation. This view includes theories of speed!
Organizational: Different departments focus on one of the quality approaches, so the challenge is to create an across language! (This is why one single approach might be easier).