Aside from major features like Personal Hotspot and iTunes Home Sharing, iOS 4.3 also brings some subtle changes to the way message alerts work. The settings for the Messages app now allow you to have alert tones repeat up to ten times at two minute intervals, which will come in handy if you're away from your iPhone for awhile and miss a text.
Most of the 17 new (iPhone 4-only) text tones introduced in iOS 4.2 have been retooled. These new tones were quite lengthy before iOS 4.3, with some of them seeming more suited to a ringtone than a text message alert. Eleven of the tones have been dramatically shortened and/or sped up -- including Noir, which I thought was short enough to begin with -- while six tones remain at roughly the same length they were before iOS 4.3.
Most tones are now less than a second long, and they sound much less overdriven, too; several of the new tones were so loud in iOS 4.2 that they sounded distorted through the iPhone 4's speaker. That's no longer an issue now as far as I can tell.
Another subtle change that you might not even notice at first: vibration alerts for messages have been changed up. Before, new messages had the same brief vibration as the new mail alert. As of iOS 4.3, new text messages will instead give two sharp and quite noticeable vibratory pulses.
We're unsure if the new tones have made it to the iPhone 3GS as of iOS 4.3; they weren't available on the older iPhone in iOS 4.2, so chances are they're still iPhone 4-only.