Re: DL video
When you start your first rep, the bar is too far forward on your foot (should be in the middle). However, after your first pull, it magically ends up over the middle as you brought it down in contact with your legs (compare :05 with :13 to see this). But then again at other points it's not really over the middle completely, which leads me to believe your pull could be straighter and more consistent.
Also, to expand on what HalfSlant said:
Your upper back doesn't seem set or tight on any reps, but maybe it's just kyphosis or w/e that is called. Your lower back is maybe set on some of the later reps. You really need to concentrate on getting everything tight before your pull. The fact that things don't look super tight or straight before you're pulling is a sign doing this will really help.
Definitely lower the bar faster. You're lowering it more slowly that you're pulling it. Just keep your breath and your back tight and lower it down. Don't be careful, just do it and get on to the next pull.
(I have/had all of these issues when DLing the last few months, except not rotating videos on YouTube
)