I'm a little surprised, but not overly stunned with Devin McCourty being taken at 27th.
Firstly, Richard Hill picked it in his profile, so McCourty was definitely in the frame.
Secondly, the AFC East has significantly upgraded the WR threats in the last few weeks - Brandon Marshall to the 'Phins and Santonio Holmes to the Jets. In other words, those teams that had decent-but-not-amazing players at #1 and/or #2 now have great players at #1 and #2 and decent #3 WRs. Considering how often Wilhite or Wheatley struggled against the #1's from last year, now they'll struggle against the #3s from this year horrible, unless something was done to upgrade. And upgrade it was; a guy who's far bigger than Wheatley and far quicker than Wilhite. He's also a solid CB and not a ball-hound liability, and seems solid enough against the run, which never hurts. As Richard said:
McCourty is a perfect Patriot player- he's intelligent, he's versatile, he's competitive and he's athletic.
Thirdly; value. McCourty's not only a CB - he's a Punt Returner (the Pats need one; Welker's hurt, remember?); Kick Returner (need one of those, too - Welker's hurt, Tate finished 2009 on IR); and he's a very good gunner and coverage unit man (can always have those, right?). Hoodie loves his special teamers, and McCourty will contribute on all special teams units immediately - that's a lot of use.
Fourthly - snap count. People were stunned that Hoodie would invest in a guy who'll be a slot corner (or as some put rather derogatively, a sub). Yet the Pats played over 50% of their defensive snaps last year in the sub packages; in other words, their 3rd and/or 4th corners were on the field most of the time. Add in McCourty's special teams snaps as returner and gunner, and they'll get a lot of use out of him, immediately.
Fifthly - what I'll call "bust-proofness". The Pats balked at picking Demaryius Thomas at 22 and traded to the Broncos; they obviously didn't see the value in a guy who hasn't played in Pro Style passing game. The Pats balked again and traded the 24th to the Cowboys, who picked Dez Bryant - the Pats avoided a potential discipline headache. They looked down at 27, saw Kindle, Odrick, Kyle Wilson, Jerry Hughes, Jahvid Best still on the board... and opted for McCourty. What's McCourty got that the others don't? McCourty's a ridiculously flexibile player. He's a CB, Nickel-back, Punt Returner, Kick Returner, Gunner and general coverage man. In other words, even if he doesn't work out as a CB, they'll find a use for him. In that way, he's much like other skill position plyaers they've taken in the high rounds. First-round pick Laurence Maroney was somewhat bust-proof because he could return kicks; second-round pick Deion Branch could return punts and kicks in his rookie year; second-round pick Darius Butler can return kicks; second-round picks Chad Jackson and Bethel Johnson were kick- and punt-returners. While it's not actually going to prevent them from being busts - nothing can - at the very least Belichick knows he's going to get something out of the pick, which he couldn't say about Kindle, Hughes, et al. A bust who can return punts is better than a bust who can't.
Sixthly - there was a run on CBs going on. I could 5 or 6 taken in the first round (depending on whether you believe Gruden that Earl Thomas is a Safety-slash-Cornerback). That's emphasised by the fact that the Jets took Kyle Wilson two picks later, despite the fact they already have Darrelle Revis and Antonio Cromartie. In other words, if they wanted a CB, they had to do it soon - else they'd be gone. Compare that to the OLB stocks, which were, to the surprise of many, barely touched - Kindle, Sapp, Misi et al are all still in the draft pool.
Finally - the second round picks, and the other 5 billion the Pats have available to trade around the draft board. People were shocked that the Pats didn't take either a DE or OLB already, but now Hoodie has all night to woo himself around the draft board in order to address his pass-rush. Kindle's there, so is Sapp, as well as Misi, and even Everson Griffen. If Hoodie wants one or more of them, he's got plenty of time to manoeuvre around the board to get them, and plenty of ammunition to do it with. However, if he wanted a good CB, well, they're all pretty much gone now.