Despite the extensive beta testing, Apple really dropped the ball with iPhone OS 3.0. A lot of people have been complaining about the many problems involved. So much so that they're already pushing 3.1 out the door; it's currently in semi-public beta. I'd thought I'd list off a couple of the problems I've noticed with it.

Inaccurate GPS
A lot of users have complained that the GPS system on the 3G is no longer accurate after upgrading. Some claim it can be a couple hundred meters up to miles off of where they actually are. It's not the GPS chip that's gone bad but rather how the iPhone calculates your location. It uses two ways of doing this. When you're outside, it uses the GPS satellite signal; if it can't find the satellite it uses triangulation. This second method measures your distance between the three nearest cell towers to approximately judge your location. 3.0 has broken triangulation on my phone but is still accurate when using the GPS. The free MotionX GPS Lite app will tell you which location method your phone is using.

Wifi Connection
Spotty wifi connectivity seems to be another issue. My iPhone will frequently claim it can't find my household wifi connection even though my laptop it's sitting right next too it shows full bars. Granted my office is on the second floor and the router is in the basement, but that never phased my iPhone before.

Overheating
The 3G seems to run noticeably warmer with 3.0, though not anywhere close to as warm as some 3GS phones get. Battery life also seems shorter, particularly when using 3G. I'm not sure if 3.0 has caused that or if it's just my imagination.

Slow Response Time
2.x felt a lot more snappy than 3.0. There seems to be a lot of lag when using common apps. Obviously Apple has optimized it for the 3GS, which sports a faster processor. Perhaps releasing universal updates isn't the best idea.