Most annoying is that there is almost no differentiation between active and inactive windows. By default, active windows have a slight suggestion of a blue glow around the edge and ever so slightly darker text and markers in the title bar. I didn't notice it on my test machines because they aren't my day-to-day computers with 3+ monitors and tons of stuff running. Now that I've got stuff spread out on my laptop with two external monitors, it really stands out.
Second most annoying is that I've got 10 Win7/Win8/Win10 machines and the only one that was ready to install this morning was my main laptop. I really didn't want to use that as my test system but I also didn't want to wait who knows how long until one of my 4 Win7/Win8 disposables is allowed to update.
Third most annoying is that one of my older computers had an old Nvidia card and The Report says Win10 can't be installed because there are no Win10 drivers for that card. So I took the card out and it still says Win10 can't be installed because there are no drivers for the card that I took out. I found instructions for forcing a recheck which says it could take as long as 15 minutes to re-scan the computer. That was 5 hours ago. The timestamp on the report is unchanged. I guess I just have to wait until it decides to scan again on its own.
Fourth is that the update icon disappeared from one of my Windows 8.1 with Bing devices. It was there a few hours ago but not ready to update. Then it was gone.
So I've got one complaint about the product and 3 complaints about how it's being rolled out.