Usability 
Hotel booking sites are traditionally the usability black hole of the mobile web. Finding and booking a hotel at the last minute is something every traveler occasionally has to do. The mobile web seems like an ideal way to do this, yet the major booking sites and hotel chains do not seem to have much interest in capturing this enormous opportunity. Most mobile hotel sites appear to be thrown together with no real usability testing or understanding of the unique nature of mobile based data entry.
Hyatt's mobile is better than most. You can search for, view and cancel existing reservations. A neat thing I haven't seen before on a mobile hotel site is the ability to check in or out online. Both these functions are well designed and require a minimum of data entry, just your reservation number and name. Members of Hyatt's Gold Passport loyalty program can check their account details and reward points balance. It's even possible to enroll in Gold Passport on the mobile site.
To me the most important feature of a mobile hotel site is the ability to find and book a hotel room on the road quickly and with a minimum of hassle. Booking a room is usually the most problem ridden area on hotel sites. The root of the problem is the requirement of hotels for full credit card information at time of booking, which in turn means completing an endless form. I have to wonder why credit card companies haven't implemented a mobile PIN system like PayPal and Google Checkout use. It's so much easier to make purchases with PayPal or Checkout on mobile sites with just your phone number and a PIN. For that matter, why don't hotels accept PayPal?