Depending on your client audience, user interfaces might not change enough for them to really feel too comfortable switching -- for example, if even the theme of my father's Windows installation changes, he has a lot of problems getting work done.
If you have a web app that looks identical to a Windows/OS X desktop app, but behaves oddly, it will irritate your customers in subtle but significant ways.
But isn't that an expectation problem? It doesn't behave as expected. But visually, it could have looked like anything. As long as the second thing looks exactly like the first but behaves differently, it will be irritating. But the first thing can look like anything you want and it'll never be annoying, unless it too looks like something else. Plus, someone who gets introduced to the second interface first will have that interface become their reference point.
If you have a web app that looks identical to a Windows/OS X desktop app, but behaves oddly, it will irritate your customers in subtle but significant ways.