Recently, I signed up to comment on a Youtube video. As I recall, it was too simple. I swear I just entered a Username and I was able to post! Note: There was no evidence of Google’s influence when I set up my “account”. Now, I wish to post comments to another video, and I keep getting a Google window that wants me to sign-in by email. Isn’t there some way to sign-in by the Username I created? [FYI: The only difference I see (and, it should not make a difference) is that I was at home on my desktop when I signed up so easily. Now, I am on my laptop. True, my OS are different, but the device nor OS should make a difference, right?]
I’m pretty sure you need a Google Account in order to log onto Youtube to comment. In the old days you just used a Youtube account, but those have long been merged with Google.
All I can think is that you were already signed into Google the first time. Google will allow you to create a username if you want instead of just using your real name.
From then on, signing into your Google account (which uses an email address) will automatically sign you into your YouTube account, though you can use the icon at the top to create another user name or to actually use your real name.
For a couple of years, Google had a project manager who wanted all Google’s different sites to be linked together in such a way as to make your real identity as your username across the board. Most people resisted, as until then made-up usernames were fine, and YouTube especially had a history where this level of anonymity was preferred (unlike Google+ which, as a social networking site more like Facebook, lended to that more logically - though there was still resistance from some).
That Google Exec (Vic Gundotra) has gone now, and immediately afterwards Youtube was less deeply integrated with the other Google sites, largely back to how it was, though a few legacy elements are still in place, one being you do need to have a Google Account. Your username, though, can be whatever you like.