I would also like a way to disable this. Like @harinosho, I download a majority of my videos to play later locally and it's just an extra step to have to pause the video. It's also annoying that having it play for that 1 second makes it part of my Continue Watching carousel.
1) Make the download button visible somewhere on the page even when the video is playing. 1b) Get rid of the slow animation of the different quality settings showing up when downloading videos.
2) Bring back the text that shows who uploaded the video. I prefer watching East Coast videos so it's annoying having to watch the first few seconds of the video to see who's on it.
3) Yesterday I had a video autorun. There should be a visible setting to turn that off. I'm guessing it was the way it was linked since the autorun flag in the URL was set to 1, but there should be a universal setting to ignore that if you have it turned off.
4) Maybe I'm missing this, but is there some sort of notification that shows if someone replied to you in a comment or forum thread?
If you're going to get a Chromebook get one with a touch screen and make sure it'll support the Google Play Store in a future update. There are some Google Play apps that should fill in some of the gaps compared to a Windows machine.