UPDATED:
18) In Photos App, Zooming In and Out is bad.
19) Once photo is opened, the image becomes clear and then again becomes blurred
16) No icons in quick status buttons for Outlook,Messaging
17) While Resizing, Back-button doesnt cancel it..
18) In Photos App, Zooming In and Out is bad.
19) Once photo is opened, the image becomes clear and then again becomes blurred
.....
It is well-known that Window 10 for Phones preview will come with bugs and we have to play role of responsible “Windows Insiders” and give the feedback to Technical Preview team. So here are some nasty and some less nagging bugs we found on our Lumia 730 running Windows 10 for Phones.

1) First the nasty one: doesn't display the background that i chose in settings. Though while switching between notification panel I get to see a glimpse of my background (for like 1 second).
2) Outlook can’t send mails and always shows some server error.
3) The Action Center scrolling is not smooth and there are noticeable lags while expanding and collapsing the quick action buttons.
4) Internet explorer web page rendering is fast but the scrolling is worse than what you may have experienced on GDR1 build
5) Images transferred to device via Bluetooth disappear after “successful completion” of transfer.
6) Settings crash, especially the personalization settings crash sometimes
7) Lockscreen settings doesn't work properly
8) Can select images only from camera roll.
9) Navigation bar isn't auto hiding and no setting to toggle that.
10)No data toggle in action centre.
11) Fitbit app stopped working after the update.
12) Back button doesnt work to go up when at down on start screen
13) Searching apps in app list - click back button- There is no change..
14) Back button function to cancel doesnt work at-times
15) Internet explorer "Reading Mode" force closes the Internet explorer
And there can be many more. Do let us know what you have seen. I have posted some of the bugs using the “Windows feedback” app and hope you would also be doing the same. You can also use a shortcut to give feedback from any of the app.