I haven't had many problems recently with the js suggested earlier in this thread still installed and these rules created by the element hider in Ad Block Latitude:
youtube.com##.opened
youtube.com###dialog
Those blocked most things, but there was still an empty pop up with a picture but no text that appeared on the youtube home page occasionally, but I could ignore it and click a video behind then it played normally. Today I think I have blocked that with this new rule, I also found I could check an extra box on the element hider to give a more precise blocking rule rather than just "img" (an earlier attempted rule blocked all the video thumbnails once, can't remember what it was):
youtube.com###img[src="
https://www.gstatic.com/youtube/img/pro ... 40x400.png"]
I've never seen the Google box, but I do often use Google search and don't habitually clear cookies etc. so I get a kind of EULA Google request on their search page if I do clean the profile which I scroll down without reading and agree to and then don't see again, so perhaps that gives me a clear pass for that, or perhaps I don't use the bit of YouTube that triggers it.
The reddit link from coffeebreak has screenshots of the pop ups if anyone wanted to see what they were. I get it quite often in a clean profile, but merely deleting all the "youtube" cookies in my normal profile doesn't produce the effect. It's been a couple of days since I saw that empty box so I had to wait to try and block it.