Covered in chronological and coincidently order of annoyance.
From the get-go all did not go smoothly. iOS installed in 2 halfs after claiming the backup had failed to be reinstalled correctly. iPod worked ok, but none of my apps would function and my music was gone.
The wifi indicator just yo-yo’s all over the place. I live in a small house and have previously had 3 bars at all times anywhere, with iOS4 the indicator is often 1 bar – shooting to 3 bars and then down to 2 as you open a webpage. The signal modulates every few seconds. Does it ‘throttle’ wifi power to conserve battery life or what?
When landing on a websites/blogs homepage and proceeding to a page and then pressing back, I often arrive back at a previously cached version of the homepage and not the latest version I just came from.
I’ve had the phantom unopenable emails from January 1st 1970 [6 of them appeared at once]. This sorted itself by exiting mail and reopening.
Several apps have randomly closed back to the homescreen.
My free copy of Winnie the Pooh becomes a ‘resource cannot be found’, but then loads if you try it again.
Battery life has taken a bit of a hit as well.
Once, upon booting, the screen has flicked between lock screen and home screen before rebooting itself and restarting and being perfectly fine again.
So… yeah, [in the style of Han Solo to C3PO], not entirely stable.