As others have noted upthread, and off the top of my head...

The "FareLock" feature.

The onClose pop-up of the "FareLock" feature.

The "Upgrade" and "Increase Fare Flexibility" upsells now appear above the flight details instead of below.

New icons.

• The person icon at "Traveler Information."
• The telephone icon at "Contact Information."
• There are nifty little airplane icons (pointing right for outbound and left for inbound; one wonders if this is confusing to right-to-left cultures).

Use of imagery, particularly palm tress.

There is more use of colors, particularly grey, green and gradient blue.

There are not-so-new but still "newish" things like ad space on the homepage and the option to buy miles.

Remember, I didn't say every change was significant. Still, if even insignificant changes are not going on at UA.com it may mean something.