I have used a lot of baby wraps (have worn all 3 of my children over the past 7 years) including the moby, I wouldn't want a knit one, personally.
IMO, the Moby is great for a very young baby. After about 15lbs, it stretches too much, and you have to keep re-tying it. Then a woven wrap, or a different carrier, works better.
However, if you were to knit one, diagonal stretch is best (most baby wraps are woven, and have this diagonal stretch) followed by vertical stretch - horizontal stretch is awful, and will be mostly unusable (you'd have a heck of time tying it tight and secure). I have not knitted enough yet to know to knit without stretch, or minimal stretch, but maybe someone else can chime in.