Performed a clean install of Avast Free 2015 RC 3 yesterday and haven’t received any popus notifying me that the defs have been updated. The defs are updating okay in the background (plus streaming is working too), but not getting any popups. The only other active protection I have running is McShield.
Dell Vostro 3700
Windows 7 Pro SP1
I have observed the same here.
I have disabled Auto Update notifications (so I don’t get any other piggybacked notices) on this system that I’m testing the RC3 version. So I can’t confirm, but I’m sure if it is widespread others will post in this topic.
I will switch it back on and see.
I just switched off ‘silent gaming mode’. 
Just checked and “silent gaming mode” is unchecked. Was receiving update popups just fine with Avast 9 so I believe this is a fault with Avast 10. It’s no real biggie as I know Avast is functioning properly in the background, but the popup notifications should work correctly.
Another thought on Avast 10: It would be nice if I there was a way to navigate directly to the forums from the main UI screen like I was able to do with 9.
An option that definitely needs to be put back!