Uing HTML5/CSS3 now, long before the specs will be completed with things like the HTML5Shiv, or my favorite, Modernizr.
Because of the ever-increasing popularity of frameworks like Django/Rails, it's very easy to write your API first, then dogfood it back onto your main website and mobile apps. It's the best way to quickly identify flaws in your own API and constantly be striving to improve it. Some companies are even choosing to get their mobile development done before their website even completes.
Connectivity is huge now. Whether it's not needing to roll your own user registration or auth system anymore by just using OpenID, or keeping your users connected with their favorite social sites while still on yours through OAuth, your website is no longer mutually exclusive from the rest of the internet. Average users almost expect you to be social and integrated.