I thought I'd take a look at the main differences between Lightroom 2, 3 and 4. Below is a comparison of all the features between the three versions.
via: Buying guide | Adobe Photoshop Lightroom 4
| Lightroom Feature | 2 | 3 | 4 |
| Location-based photo organization | * | ||
| Importing libraries from other software | * | ||
| Extended video support | * | ||
| Basic support for video files | * | * | |
| Improved watermarking | * | * | |
| Cross-platform 64-bit support | * | * | |
| Multiple monitor support | * | * | * |
| Offline photo management | * | * | * |
| Batch application of Develop settings | * | * | * |
| Adobe Photoshop integration | * | * | * |
| Develop presets | * | * | * |
| Tools to manage and refine many images at once | * | * | * |
| Comprehensive image management | * | * | * |
| Multiple viewing options | * | * | * |
| DNG conversion | * | * | * |
| Single or group metadata stamping | * | * | * |
| File renaming | * | * | * |
| Advanced search and sort | * | * | * |
| Simple, customized keywording | * | * | * |
| IPTC/EXIF/XMP metadata support | * | * | * |
| Timesaving presets for import/export | * | * | * |
| Metadata stamp on import | * | * | * |
| Highlight and shadow recovery | * | ||
| White balance brush | * | ||
| Additional adjustment brush options | * | ||
| Advanced noise reduction | * | * | |
| Film grain simulation | * | * | |
| Lens correction | * | * | |
| Perspective correction | * | * | |
| Selective editing brush | * | * | |
| Nondestructive environment | * | * | * |
| Superior image processing | * | * | * |
| Tracking of editing adjustments | * | * | * |
| Before-and-after comparison | * | * | * |
| Advanced black-and-white conversion | * | * | * |
| Simplified tone curve tools | * | * | * |
| Advanced hue, saturation, and luminance editing | * | * | * |
| Intelligent sharpening | * | * | * |
| Dust-busting | * | * | * |
| Red-eye removal | * | * | * |
| Photo book creation | * | ||
| Video publishing | * | ||
| Soft-proofing | * | ||
| Direct email support | * | ||
| Enhanced online sharing integration | * | * | |
| Improved customizable print packages | * | * | |
| Portable, easy-to-share video slide shows | * | * | |
| Flexible online publishing directly from Lightroom | * | * | |
| Integration with Flickr, a Yahoo! service | * | * | |
| HTML or Adobe Flash Player compatible web gallery output | * | * | * |
| One-click publishing to your website | * | * | * |
via: Buying guide | Adobe Photoshop Lightroom 4