Thanks, and yeah I was gonna use that spotlight style subscription link annotation.I think it's simple and to the point! Make sure you're using YouTube annotations to make your subscribe button interactive.
Also, you may want to consider overlaying some of your other popular videos on the end card. Then again, it all depends on what you want to do with the card itself (perhaps you just want to keep it clean).
I agree with your first point because I'm a bit OCD about things like that. I didn't have any way to center it in Photoshop (that I know of) so I just eyeballed it as best I could.I think it's great! Like AcademiCode said, It's very simple and very clean. I really like that in my graphic design.
A couple points, though. The first one is kind of nitpicky, but the information on the slate, the "thanks for watching" as well as the buttons, aren't lined up in the middle. There's more empty space above than there is below. Secondly, I noticed that the image is 1280x720 pixels. Maybe that's just this version of it but if not you might want to make a 1920x1080 pixel resolution one if your video will actually goes up to 1080p