This sounds like an issue with working within the users' expectations. The whole "glass buttons v non-glass buttons" is a moot point. This argument based on aesthetics.
However, the there is usability question regarding the document style: "What's wrong with using a slick, modern style?"
Well, it's all about user expectations.
When I look at the glass button, I think Apple because most Mac apps use the standard Mac styling, which includes glass buttons. And, if you are on a Mac, you would expect to see these buttons. However, it would be jarring for a Windows user to see standard Mac styles in his windows environment. Or if on a intra-net, why change the style for just one form?
Why make it harder for users to use just your product? Remember, users spend most of their time on other websites. Don't make them put in more effort for your design.
There is a good reason for congruency. It's an easy and effective way to ensure that your users can recognize elements.
More importantly, it draws attention way from your buttons (or other distracting, "shiny" element) and allows the content (which should be the main focus) to shine through.
A site doesn't have to look pretty to be useful and provide a good experience.
Don't spend time jazzing up boring intranet forms, it won't fool anybody. Just work on making the content and process more-user friendly.