Weather animations have been completely redone, with 3D maps you'll be proud to have just sitting on the screen when your iPad or iPhone isn't doing anything else. You can see the weather in multiple locations at once, and get push alerts (USA & Europe) for severe weather. The weather data comes from Weather Underground.
Weather HD 2 adds some social features so you can share your weather with friends or see what's happening in their cities. There is also a multi-paneled display showing animations of all your saved forecast locations on one screen.
You can customize detailed info for each location, adding things like dewpoint, sunset and sunrise times to make the information as dense or as simple as you would like. There are nice little touches like animated waves on the coasts and animations that put radar in motion.
If you could only choose one consumer based weather app I think this should be the one. Of course, there are a few things to nitpick about. For example, one mode of the display shows weather-based Twitter comments that pop up at random on the map. I personally can't think of a more worthless feature. Happily, it can be turned off, because it's just a distraction.
The app also allows you to buy forecasts from additional services like Accuweather and Meteogroup. I really dislike in-app purchases, but fortunately you are not forced to do so, as I think the Weather Underground data is just fine.
Weather HD 2 improves upon an already excellent weather app. The price of the app is reasonable considering the wealth of data it contains and the sharp presentation. There's also a free ad-supported version that limits the number of cities and omits push notifications. I'd skip it and invest the buck.