Oh, I see. I agree with this. It never occured to me that the slider pad on the ID would bug me, but I've never used a bag quite as much as I've used the ID, either. And I find that the adjustment of the strap length does slip in teensy tiny increments, too, until I'm suddenly perplexed as to how my strap got so long and have to readjust. But this isn't a serious complaint, it's just, as you said, the weakest feature of a fantastic bag.
On the ID bag, which is a great design overall, I'd have to say that the strap, relatively speaking, is the weakest feature. I don't mind having to adjust this (the first time) at both ends, and the width of the strap is a plus, but the straight, slider shoulder pad is less than optimal.