1) No micro SD card on 10581.
2) Cortana will not perform voice commands as in 8.1 ie: "Cortana, play Billy Joel Glass Houses." it just displays a Bing page. She will open Groove, but will not cue music.
3) Cortana is irratic answering texts by voice when listening to audio on BT headset. Sometimes she cannot understand what I sat at all and sometimes she does not read them, and sometimes she does not resume audio that she interrupted.
4) Cannot set custom ringtones, this is the second time they broke this.
5) I still do not see syncing between PC and phone like color, theme etc.
6) If you pause an audio track on Groove with BT headset, it will not restart unless you do it in a short time, with Music or Xbox in W8.1 and W7.5 it seems to me the paused song would resume on play button press even an hour later, (provided the BT headset wasn't shut off)
Can't think of others off the top of my head, but there are other quirks for sure.
The battery life has been great, just did a long trip, 18 hours and still at 35% using for audio, BT, and some web browsing.
I really like outlook and calendar. (They sync well)
HOPE ALL THIS PLUS MORE IS FIXED
(HTC One M8)