He's a first round pick who is being thoroughly outplayed by Dane Fletcher. That's not a good sign for him. I'm still hopeful that he'll perform better when he's playing in a role that's more suited to his talents, but at this point he hasn't shown the ability to do anything especially well. I understand the angst, although I agree that people need to be patient with him.
I also think it's a problem of expectations, in part. The general consensus on this board was that he was more pro-ready than Chandler Jones was (I'm going to take a moment and pat myself on the back here, since I thought the opposite). While that didn't turn out to be true, he did show flashes of real, disruptive talent as a rookie. He seems to have regressed from that this year.
I have a couple of theories for why that apparent regression has happened. The first is that his greatest weapon is his versatility. While I think he might be miscast as a 4-3 OLB (particularly on the weak side), he can play credibly as a MLB, 3-4 ILB, and can even take some snaps as a 4-3 DE (or DT in rare situations). As a rookie, he was given limited opportunities to rush the passer, and he made the most of those rare opportunities with 4.5 sacks. That's what's missing this year. I think the injuries across the defense have forced the Pats to play a more basic defense across the board, and the result is that a guy like Hightower, who derives a lot of his value from being versatile/unpredictable, becomes less valuable. Conversely, a guy like Chandler Jones, who simply wins his matchup over and over again, maintains his value.
In an ideal situation, Mayo and Wilfork come back next year fully healthy (or something close to it), and Collins has fully emerged as a swiss army knife in his own right. The end result is that you have a ton of rotating parts who can play all kinds of positions. A front seven that includes Mayo, Ninkovich, Hightower, and Collins should in theory be able to send the pass rush from basically anywhere without telegraphing what it's doing, since all of those guys have the skill to drop into coverage without making total asses of themselves if the rush comes from somewhere else. I would really love to see it play out that way, because it would be something else to watch.
It also seems like Hightower wasn't handling playcalling well. When Mayo went down, Hightower took over the green dot, and I think the added responsibility really took him out of his game. IIRC, the coaching staff seems to have felt the same way, since I believe he's now been stripped of that responsibility.