For a long time, people accused Apple for not doing the right thing by keeping the privilege to develop iPhone and iPod Touch applications only to themselves. Now, it seems they were right, although this may change in the future, and I am almost sure it will. The most frequent problem reported by iPhone 3G users is the tendency of third-party applications to crash, sometimes even on launch, while Apple's own programs work like a charm.
Leaving the third-party applications aside, it seems a large number of iPhone 3G owners are facing interface lag, but resetting only temporarily fixes the problem, and this issue also seems to be present in old iPhones.
While this is not something new, and was noticed by one of the first iPhone 3G reviews, a lot of users are experiencing echoing audio on their side when pairing up the iPhone with a Bluetooth audio system, especially in cars.
According to some additional reports, GPS mapping doesn't work as expected, in some cases, but this doesn't seem to be a widespread problem. Anyway, it's funny to mention that a British iPhone 3G owners found himself located off the coast of Canada...at least, according to his iPhone 3G GPS, of course!
At last, there were also a few reports saying that the ringer volume is resetting without any user interaction, and individual preferences are turned back to default. Obviously, there are rumors regarding the fact that Apple is already working on the iPhone 2.0.1 firmware update, to address some of the problems described above, but nothing is official yet.