First let me preface that I am aware this is a unfinished, preview OS. With that said... I too rolled back to 8.1 after months of struggling with the latest 166 and 512 builds.
I will say that the developers have been doing a great job and bringing Windows 10 Mobile up to snuff, but its the little things that pushed me away. The two second delay for the clock and notifications to show up when pressing the power button, come on guys really? Its not like this is an alpha build, why are we reinventing the wheel and breaking solid components this late in the game?
Loading........
Other things like photos not auto-uploading to OneDrive... which was once an easy task in 8.1 is utterly broken in 10. Having to manually upload photos to OneDrive is taking the "smart" out of this phone. Even with setting this up in OneDrive, I could not get one photo to upload with success. Yes I know that it's a preview app on a preview build (and I am sure it will be fixed at some point), but its a serious flaw that would prevent me from upgrading to 10 in the future if not corrected.
Resuming....
Another item keeping me from using Windows 10 Mobile is how they handled sharing content like photos or videos. In Windows 8.1, you could easily share a photo or video and it would auto-compress the media based on how you were going to share it. In Windows 10, good luck sharing your uncompressed 72 meg video file by email.... I would have to manually set the encoding before recording a video or grab the video over USB, upload to OneDrive and share uncompressed. I would rather "set it and forget it" (Ronco-style), leaving me the option to auto-compress later when shared. This missing feature is a deal-breaker coming from 8.1, and sadly if they don't plan to implement the full 8.1 feature-set into Windows 10 Mobile... I can't justify upgrading. I personally do this so often I couldn't bear using 10 any longer.
There are several other issues I encountered like SIM issues (losing mobile data), battery drains, random crashing, videos stutter during recording, unresponsive touch screen input, pinned websites redirect to the number 3 in bing search, app installation loops, and live tiles not updating. Just some of the few issues encountered over the past two builds, all of which I submitted feedback for. I experienced enough issues to seriously make me worry about the finalized version we'll actually end up. Thus also made me start thinking seriously about other platforms.
Then I snapped out of it, continue to tell myself "its just a preview build, its just a preview build... right?"