Here's what I HATE:
* No Podcasts app. I like to listen to Podcasts in the gym and want to leave my phone in the locker.
I can get around this by changing the Media Type of the Podcasts to "Music" in iTunes, dumping them into a Playlist, and then syncing this to the watch with my phone. This is stupidly long-winded for what should be stupidly easy
* Bluetooth setup is HORRIBLE. I use a pair of Plantronic FIT. For the above to work, I need to swap the Bluetooth pairing between headset > phone, and headset > watch. This seems OK; the watch is able to grab the connection simply by clicking on the device. It falls apart when I try to re-pair the headset with the phone. I have to stick the watch into Airplane mode, just to shake off the connection to the headset, so the phone can grab it back.
TBH, the way the iPhone handles Bluetooth devices has always been ghastly. There's no way to disconnect the devices, you're simply forced to switch off Bluetooth or "Forget the device". But throw the Watch into the mix *which relies on Bluetooth to do 90% of things* and you have a huge mess. "Disconnect Bluetooth Device". Is that so difficult, Apple?
Switching Bluetooth headsets between two devices that are interlinked seems counter-productive anyway. A good idea would be for phone to be able to use whatever the watch is paired to output sound, then there would be no need to switch between the two.(Since the devices are co-dependent, nothing would be lost).
These are by far my biggest bugbears so far.