Of note would be some of the HTML5 input types. As far as I know any browser that doesn't recognize a type on an input element, defaults to type="text". The added value is minimal, but it's worth being aware of.
On devices running the iPhone OS for example, an element like:
<p><label>Telephone: <input type=tel></label></p>
or,
<p><label>Email address: <input type=email></label></p>
would bring up an appropriate keyboard, giving importance to characters frequently needed in the respective input.
Given the immense semantic and user experience value, and the graceful degradation, I've adopted this on production sites without detriment. In some cases, especially with mixed doctypes, you could lose out on validation, but that's to be expected in a transitional period, and a sacrifice I'm willing to take.
A lot of good info on the W3C site, regarding the updated form elements and specs.