This is by no means a comprehensive review of the iPhone OS 3.0, but rather a first impression.
Cosmetically, there isn't much difference. What you might immediately notice is that a new Voice Memos app is forced onto your front page. If you had that page full, it
kicks the last app to the second page with ONLY that app. The third page continues the rest of your apps. For a while there, I thought it had deleted all my apps from the second page. Thankfully, that wasn't the case.
Speaking of the Voice Memos app, I found it to be a pretty cool app even though I'd probably not use it much. It's easy enough to record and play it back. There's a nice VU meter that enhances the experience. What's actually impressive is that you can trim the ends of your audio memo. It's very intuitive, just like the iPhone 3G S video trimming. You can then e-mail your memo, send it through MMS (when it's available) or sync it with your computer through iTunes.
If you swipe to the left on the home screen, you can use Spotlight Search to search for just about ANYTHING on your iPhone. What it's really useful for is to get to that app that's on your 11th home page. If you're on the first home screen, hitting the home button will also take you to Spotlight Search.
The feature I was looking most forward to was push notifications. This allows apps to notify you of an alert even when you're not in the app. Now, the Notifications setting doesn't automatically appear in Settings for 3.0. To enable it, you will need at least one app that utilizes push notifications and enable it in that app. So far, I have found one app that I already have that uses it: AP Mobile. It's free, and it's a good way to get yourself familiar with push notificiations. AP Mobile pushes breaking news to the screen of your iPhone. I haven't received one yet, but I'm hoping I will soon.
The overall navigation of the screen seems to be slightly faster, although some have reported lag. Usually a hard reset of the iPhone should fix that.
One little-known feature of 3.0 is the "HTTP Adaptive Bitrate Streaming." This makes live video a better experience on the iPhone. The first app to make use of this is MLB.com At Bat 2009. Starting tomorrow, certain games will be televised LIVE on your iPhone! The games up will be the White Sox vs. Cubs and the Tigers vs. Cardinals. I'm stoked since I'm a big Cardinals fan! This really puts MLB.com At Bat far and above ANY other sports app on the App Store. This app has already featured live streaming audio, saved videos of the major plays of each game, live pitch-by-pitch summary and a large amount of stats which will make your head spin. Super good deal for $9.99.
UPDATE: Actually a couple of games were televised live on the app on Wednesday. I caught the Dodgers vs. A's game but there was no sound and the video was crappy for Wi-Fi. I'm not sure this is indicative of how it will be tomorrow when it is supposed to launch officially.
If you want a free sports app, try out the U.S. Open Golf Championship app. It also starts on Thursday and features live video. If it's anything like The Masters app, it will be a great experience.
As I go through the different 3.0 features, I'll be posting my finds here.