I do not think Harris gets anywhere near the credit he deserves. Jets fans have heard the term "core of four." They think of the other three, D'Brickashaw Ferguson, Nick Mangold, and Darrelle Revis as elite at their positions. Harris does not get as much love. Part of it stems from the fact the other three can prove themselves consistently lined up against elite players. Harris kind of plays all over the field without a consistent matchup opponent.
I also think David suffers from a bit of Curtis Martin syndrome. Nothing he does on or off the field is flashy. He doesn't have incredible speed. He isn't making a ton of impact plays like sacks. He isn't a crushing hitter like James Harrison. He also isn't vocal with the press, which could draw attention to himself in other ways. All he does is consistently play well.
Make no mistake about it, though. He is an elite inside linebacker. He might not be as good as a guy like say Patrick Willis, but there is a difference between best and elite. Most teams would drool at the chance to grab him. Fortunately, the Jets know what they have. I still remember Rex Ryan laughing at the thought of him trading Harris back in 2009.