The Touchy Antennae
Apple may have addressed the apparent "reception issues" with the iPhone 4, claiming that it was due to a software bug that caused inaccurate reporting of signal strength displayed as bars on the all the iPhones released to date.
Of course, changing the software readout won't change that the iPhone 4's antenna is easily attenuated by simply holding the phone. AnandTech's sharp testing found that the iPhone 4 will still drop 24 dB in signal when covering the lower left part of the antenna. If you're standing right next to the cell tower, a drop in 24 dB isn't a big deal. If you're within 24 dB of dropping out of range, gripping that iPhone 4 will mean No Service.
The software may not be reporting the signal strength correctly, but that won't fix what is a hardware design issue. Even the folks working at AppleCare have been told that the pending software update won't do much to resolve real world reception issues. Check out this neat video from TUAW showing what the "death grip" does to reception, regardless of bars or not.
The Yellow Camera
The iPhone 4's camera is one of the best in mobile phones, thanks in large part to its illuminated CMOS sensor. The iPhone 4's outdoor and brightly lit shots are brilliant, but some owners are finding that indoor shots don't have the correct white balance set, making images yellow. MacWorld tested the iPhone 4's camera up against previous iPhones and a D-SLR, and something is definitely up. Hopefully it's not a hardware issue.
Buggy Proximity Sensor
Another issue reported on the internet is a buggy proximity sensor. Touch screen phones today have a built in proximity sensor that will turn off and disable the screen when the phone is in contact with the face. This saves power when the user is talking on the phone as well as prevents unintended phone operation and dialing from the cheek.
With the previous iPhone models, the proximity sensor was located to the left of the earpiece. Now in its place is the front-facing camera, so the proximity sensor is now moved further up. iPhone 4 owners are finding that the sensor or software may be faulty, leading the screen to blink on and off rapidly. Read more on Macnn.
Capped Upload Speeds
This one isn't really the fault of Apple, but rather of an exclusive network that the iPhone 4 is currently running on (maybe?). Apple never made any announcement that the iPhone 4 would have HSUPA as well as HSDPA – a big upgrade over the previous 3G models that had only high-speed downlink. Early tests confirmed the big speed boost when uploading, but now it seems that AT&T has capped that uplink speed in certain markets. UPDATE: AT&T now claims that the slowdowns are caused by a software issue and that it is working on a fix soon.
Forum users on MacRumors are reporting that their once phenomenal upload speeds are now unable to get anywhere past 100 kbps.