There's nothing worse with the full adoption of a web standard than the lack of interoperability between applications. I'm currently developing a new web site, after finishing this, and I have to say that this site works even on Solaris with Firefox/Mozilla. But this is an exception.
The sad thruth about interoperability lies in the fact that this word doesn't necessarily mean "you have to implement a standard so that your implementation is fully compatible with other implementations". This is the current point of view of the W3C, not the point of view of some companies. Some browser vendors tend to implement a standard in a way that only some developers are able to follow, that is, only those developers who use and adopt their technologies.
For example, XML's implementation is currently interoperable between Firefox, Opera, Safari and Chrome. Internet Explorer, instead, decided to implement it through the MSXML library, that works together with Trident. By doing so, even XSLT, SVG, MathML are supported only if you install a special plugin that is able to connect itself to MSXML. The following tests of mine, XML and DOM tests and XML and CSS test, show clearly what I mean: the overwhelming majority of them fails spectacularly in IE.
We need to wait. I think that only time will tell if the word interoperability lies in a vacuum or is something more.