I wouldn't say they underachieved. I think people are just forgetting how good that 2013 defense was.
It wasn't like Revis and Browner joined the 2011 Patriots. That 2013 defense was really good too, and history may have looked quite different if Welker hadn't taken out Talib in the play-offs.
Several people have mentioned the running woes this year, but we actually were significantly improved from last season, even moreso than the passing game.
For example, here are the differences in the numbers between the two seasons.
Stat: 2013/2014
Points Allowed: 338 (21.1 per game, 10th)/313 (19.6 per game, 8th)
Passing yards allowed: 3,824/3,837
Adjusted Passing YPA: 6.0/6.2
TDs allowed: 25/24
INTs: 17/16
Rushing yards allowed: 2,145/1,669
Rushing YPA: 4.5/4.0
Total yards allowed: 5,969/5,506
I think people remember the early problems in the run game, and some of those re-appeared during the play-offs. But overall, it wasn't great, wasn't bad, and we were ranked 12th in the league in run D. The attempts to run went down dramatically too, from 481 to 419.
The passing stats are eerily similar actually, and improved from our 2011/2012 seasons (although 2012 started to get better after the Talib deal).
And as others have mentioned, like most BB teams, they got better as the season progressed. Browner came back, Hightower adjusted to life without Mayo, Collins grew into his role, Chung re-established himself, Siliga got healthy, and we brought on Ayers and Branch.
There were improvements in certain areas, no doubt. 3rd down defense was better (42.2 to 40.2). And there will never be enough sacks to appease people, but the pass rush was effective enough to bother passes, if not always get home.
And FWIW, we finished tied for 13th in sacks in 2014, one sack away from tied for 9th. Only 1 of the top 7 teams in sacks made the play-offs (Baltimore), while 5 others were in that 8-13 range with us. So it's an important thing, but not THE most important thing. Due to our system and 2-gapping, we're not going to generate the most sacks, but we did quite well in that area in 2014.
So no, I don't think our defense underachieved. I think it continued to improve, a trend that started in 2012. And I think some expectations were just too unrealistic.