One missing feature we noticed right away: Spotlight no longer searches through MMS messages on the iPhone 3G after the iOS 4.1 update. Given that disabling Spotlight searching was the most common suggested remedy for poor iPhone 3G performance on earlier builds of iOS 4, this omission is somewhat understandable.
More puzzling, however, is another omission the folks at geek.com noticed: over-the-air Notes syncing is now missing in action on the iPhone 3G, too. Up until now, the iPhone 3G, like its newer siblings, has been able to sync Notes between the iPhone and a Mac/PC via Gmail or MobileMe rather than through iTunes, allowing for near-instant Notes updates between devices. According to Apple's support document on Notes syncing, both the iPhone 3G and second-generation iPod touch have lost this feature. You can still sync Notes with the iPhone 3G via iTunes, but you'll no longer enjoy device-to-device updates without syncing your iPhone first.
Given the plethora of alternate apps on the App Store that offer far more flexibility and features than Apple's built-in Notes app, though, this omission is something we might never have noticed if one of our readers hadn't brought it up. We're not sure why Apple removed this feature in 4.1; it's possible that, like Spotlight searching of MMS, it was one of the things dragging down performance on the older devices. Either way, losing over-the-air Notes syncing in IOS 4.1 is a small price to pay for an iPhone 3G that actually functions properly in iOS 4.
Thanks to Jeff for bringing this to our attention.