Diagonal attachment - as inferior shape
Charles One way to understand the issue here is to see that Black a is a good way to play after . Therefore in general should be played now. Instead at b is a play to attack Black more strongly, but Black at a will then remain as a weakness.
That's just one aspect of the problem for White with this shape: it tends to concede some influence.
Actually, if you analyse continuations after the diagonal contact then tenuki you can see two good follow-ups.
This is a gentler and lighter way to play. White's attempts to cut it will end in bad shape.
If White tries to cut the other way, with here, Black could answer with a or b. Black at a, White at b to cut and White has an empty triangle.
And here it seems like White has more worries than Black. Usually will look like an overplay.
To add to that, Black has the option to play in the diagram above some time as a kikashi, protecting the cut.
Bill: hits the basic weak point of White's formation. It takes away potential eye shape for White and threatens to connect via a.