Comparisons With the First iPhone
The very first thing I noticed about the iPhone 3G compared to the original model is that it's slightly heavier. The dimensions are almost the same, although the iPhone 3G is slightly thicker in the middle and more tapered at the edges.
This slightly rounded form factor feels better in your hand over extended periods, though the combination of smooth plastic and beveled form might also make it a little harder to grip. My advice: Get a case to protect the iPhone if it happens to slip from your hand. As widely noted already, the headphone jack is no longer recessed, enabling the use of any standard set of headphones.
One thing that doesn't come across in photos is how striking the white model actually is. I'd gone into the AT&T store determined to get a black model because the white ones didn't look good to me in pictures and I figured it would show dirt more easily. Once I saw the white model in person, I changed my mind.
Something else you don't notice in photos is a slight difference in the screens between the new and original iPhones. Both offer excellent resolution and clarity, but there are subtle differences in color shading and black tones. The colors seem slightly richer on the iPhone 3G, though the difference is small enough that you need to place both phones side by side to really see it.
More noticeable is the slight change in positioning of the speakers and microphone in the new design. Although I haven't yet noticed a big difference in calls made while holding the phone to my ear, ringtones or application sound effects are definitely louder and clearer when played through the external speaker. Even more dramatic is the improvement to the speakerphone, which is now among the best such feature I've heard on a mobile phone.
New Features Specific to iPhone 3G
Most of the "new" features in the iPhone 3G are really part of the iPhone 2.0 firmware update that can be installed on any iPhone, new or old. There are, however, two big differences between an original iPhone and an iPhone 3G. The first is, of course, the 3G network access implied in the new iPhone's name. This is, in fact the most significant difference between the first iPhone and the new model -- and it's the most significant reason to upgrade.
It sounds cliche to say there's no comparison between the data capabilities of the two phones, but sometimes cliches are true. This is one of those times. The informal testing I've done so far has involved turning Wi-Fi access on and off -- both at home and at a handful of Wi-Fi hot spots in my neighborhood. While I won't say that AT&T's 3G service matches my high-speed cable modem connection, can say that it does keep pace with the mom-and-pop coffee shop down the block (which I relies on an entry-level DSL connection and which was being used by a handful of laptop users).