When deadlifting, I know it's recommended to keep the bar path straight [and to drag the bar up your legs.
Deadlift Bar Path Trajectory
In the Deadlift the bar path should be pulled back on top of you, not straight up.
Pulling the bar back on top of you positions it closer to your body's Center of Gravity, COG.
Pulling the bar straight up positions the bar slightly out farther from your Center of Gravity.
Torque
The farther away the bar is from your body's Center of Gravity, the greater the Torque.
Torque magnifies the weight on the bar beyond it true load.
John Kuc's 1600 lb Plus Deadlift Attempt
Kuc was on of the great Deadlifters of the 1980's. Kuc's best Deadlift at a 242 lb body weight was 870 lbs/395 kg.
McLaughlin (PhD Exercise Bio-Mechanics/Former Powerlifter) wrote a series of Powerlifting and Deadlifting article on the bar path in the Deadlift, Squat and Bench Press for Powerlifing USA magazine based on his research.
McLaughlin's research examined one of Kuc's Deadlift attempts with around 860 lbs, where Kuc allows the bar to get a little to far out in front of him.
McLaughlin determined the Torque of the bar being slightly out in front of Kuc magnified the bar weight to over 1600 lbs.
Needless to say, Kuc missed that Deadlift attempt.
Dragging The Weight Up
This provide two benefits...
1) It ensure that you are keeping the bar as close to your body's Center of Gravity as physically possible.
2) It provide greater support for your stabilizer muscles.
Dragging the bar up is similar performing a lift on a Smith Machine.
The bar gliding up "The Rails" of your legs, like it glides up the guide rods on a Smith Machine.
...once I get to my knees, I find my legs are nowhere near straight enough to drag the bar up my thighs. If I were to do this, then the bar would be traveling back instead of straight. Is this an issue with my form or just anatomy?
Traveling Back With The Bar
That is precisely what you want; to pull the bar back.
Pulling it straight up slightly increase the Torque, magnifies the true weight on the bar.
That means you need more force/strength to finish the Deadlift. It guarantees your Deadlift will be less than it could be.
Is this an issue with my form or just anatomy?
Poor Coaching
The issue comes from some poor coaching on how to Deadlift.
You body is instinctively wants to do the right thing, pull the bar back on top of you.
However, your are overriding it by trying forcing the bar to travel in a straight line, slightly farther away from your body's Center of Gravity.
Your Video
It hard to determine what you doing from the angle of the video.[/QUOTE]
Summary
To reiterate, pulling the bar back on top of you (traveling back with the bar) ensure you keep the bar as close as possible to your body's Center of Gravity and allows you to slide the bar up "The Rails" of your legs, like a Smith Machine.