You know the story. You build a Drupal site. It's amazing. Then the client asks for 2 simple features to be added. And you can't deny that they are reasonable requests. If only you'd known about them when you started!
One issue I haven't seen discussed much is the impact of touchscreens on how we design web pages. The most obvious issue is that for an iPad or iPhone user there is no such thing as "hover" or "mouseover" - in software-speak, touchscreens do not support mouseover.