Scott Kelby has written a detailed review on his blog, and both he and Matt Klokowski have put together a great series of videos looking at and explaining the features over at The Lightroom Learning Centre and also at Kelby training.

The key improvements – in no particular order are:

• Great Noise Reduction - finally Lightroom overtakes all the other options, with a pro level noise reduction capability, which is especially powerful as it works on the RAW images rather than th an 8 bit processed image.
• Watermarking tool – enhanced watermarking tool giving you increased flexibility over opacity a, size and placement.
• New Importing Capabilities – a much more intuitive and visual import process that allows you to see where your images are coming from and where they’re going
• Flexible publishing options – drag and drop capability with the ability then to publish direct to flickr, share with your iPhone etc.
• Film Grain Simulation - to enhance  your images and bring back that film look.
• Sideshow Audio – The ability to embed music into a slideshow, and for the music to be synch’d to the the length of the slideshow and then exported as a package either as video or as a slideshow.
• Imprvoed Vignetting - the main changes are to the post crop vignetting, which has been really enhanced.
• Improved Print capabilities – there is increased flexibility about how the print “grid” is laid out including the ability to have photos of different sizes in the same grid. You can also change the print background colour.
• Workflow improvements – they added the collections panel to the Develop modules meaning you no longer have to flick back and forth between the two.