According to the release notes The goal of this release is to address several bugs that were introduced with the release of Lightroom 2.0 and provide additional camera raw support. A comprehensive list of issues fixed in this release is included below:

Stability and High Severity Bugs - Fixed

• Lightroom could crash when switching to or from the web module on Windows Vista 64.
• Selecting a custom camera profile or using auto-white balance could cause a spinning ball.
• Preference files pointing to missing catalogs could prevent Lightroom from progressing past the registration dialog.
• Errors could occur when attempting to create a virtual copy with a computer that has a processor that is not SSE2 enabled. (This is a method of delivering instructions to the processor which was introduced with the Pentium 4 processor. It should not have affected a significant number of modern machines.)
• Modal dialogs could try to appear on a disabled second monitor, causing Lightroom to appear hung or unresponsive.
• When a DNG file was selected for export to a derivative TIFF file and it is added to the catalog at export there was a possibility that one of the files could be treated as a sidecar within Lightroom such that deleting one file deletes both.

Performance Bugs - Fixed

• Attempting to import a large number of images that already exist within the Lightroom catalog could cause significant performance delays.
• The image counts that appear next to each folder could appear to populate slowly.
• Lightroom scanned the calibration profile directory excessively when switching collections.
• The 'three dot' indicator in the grid view persisted longer than necessary.
• The Mac 64-bit version did not have SSE2 optimization, causing reduced rendering speeds.
• Memory leaks in the Library and Develop modules decreased Lightroom performance.
• Applying keywords via presets or on import could greatly decrease import performance.
• Folder synchronizing could experience significant performance decreases.

Keywords and Metadata - Fixed

• Keywords from Lightroom 1.4 were not included on image export by default. (Please note that catalogs upgraded from Lightroom 1.4 using Lightroom 2.1 will not have this issue, but catalogs upgraded from 1.4 using 2.0 will still need to use the script posted at our technical support site at: http://www.adobe.com/go/kb405074.)
• Lightroom stopped rendering previews prior to completion on files with metadata changes applied by other applications.
• The "Any Searchable Field" search option did not find caption text.

Photoshop Interop - Fixed

• Edit in Photoshop did not work correctly or at all for some customers.
• Copyright Status and Info URL fields were not transferred when a file was created from Lightroom for use in Photoshop.
• XMP metadata from the IDimager application was not carried over during an Edit in Photoshop operation.
• Files with an ampersand in the file name were not added back to the Lightroom Library after an Edit in Photoshop session.
• Choosing to Edit in Photoshop from a folder with an apostrophe in the name would cause the feature to fail.
• Certain NEF files were rotated incorrectly when opened into Photoshop from Lightroom.

Miscellaneous Troubling Items - Fixed

• Re-importing missing photos from an exported catalog could ignore the setting 'preserve old settings as virtual copy.'
• Choosing to undo a slider adjustment in Lightroom on Windows would cause two undo steps to occur if the slider was still under active mouse selection.
• Lights Out mode could flicker on Windows when the detail panel was selected in the Develop module.
• The crop aspect ratio could revert to the last selected ratio even when it had been reset to original.
• The spot tool might not switch from clone to healing mode upon request if the brush was above a certain size.
• Certain ORF files were not read by Lightroom.
• During import and conversion to DNG, file renaming could use the operating system's date stamp rather than the file’s EXIF date.
• Panoramic images printed with a portrait layout and utilizing the auto layout functionality would cause images to print at a low resolution.

But what do you do if you have a large number of images and you need to find out which ones are not in a collection? Here is a step-by-step way to select all your non-collected images.

While in the Library module, open your Collections panel on the left.

Now click on the first collections in the list to highlight it.

While holding down the shift key click on the last collection in the list. You should now have all your collections selected and all the images in them appear in the grid.

To select all these images either press Control-A (Command-A on the Mac) or go up to the Edit menu and click Select All.

What's that you say? You're right! Now you have all your collected images selected. Here's how we get to our goal of select all images not in a collection.

Go to the Library panel and click on All Photographs.

Now the grid shows all your images with the collected ones selected. To finish off got to the edit menu and click Invert selection.

Viola! Now the grid shows a selection of all your images that are not in any collection!