No Stereo Bluetooth
Although the first iPhone had Bluetooth support, this was limited to the monaural “headset” profile, meaning it was designed for making phone calls but not for listening to music. While stereo “A2DP” Bluetooth headsets were still fairly uncommon back then, it was an unfortunate limitation, especially in light of Apple’s weird headphone jack.
Stereo A2DP support didn’t arrive until 2009 with the iPhone 3GS and iPhone Software 3.0, and although the 2008 iPhone 3G gained it retroactively, as usual the original iPhone was left out of the party.