As I tweeted out last night (@dcrockett17), I'm a die-hard Mizzou guy and even I didn't see late second round for Britt -- I was thinking anywhere from late 3rd to undrafted. WR and OT are probably the two positions where beauty is most in the eye of the beholder once you get past the consensus top-tier talent, and Britt is most definitely a "coach's pick" more than a personnel director's pick. As others have noted, it's pretty clear that Tom Cable gets his way on the offensive line.
So, what does he see in Britt?
If we are inclined to take the newly svelte Cable at his word he likes the Giaco-like play-through-the-whistle style, the position flexibility, and I would guess that he also likes Britt's game-to-game consistency. I will try to make these points with tape mostly from 2012, where Britt played mostly RT but played LT too.
First, here is a bit of relevant background. 2012 was Missouri's inaugural season in the SEC. Long-time offensive coordinator David Yost, and Mike Leach-style spread proponent, left to join Leach's staff in Pullman prior to the start of the season. Josh Henson, a former Les Miles assistant going all the way back to Miles' time at Oklahoma State, took over as OC. The most substantial changes Henson implemented occurred along the o-line, where he tightened the splits considerably and brought the TE in-line. What remained the same, however, was the tendency to leave the OTs on an island in pass protection, opting instead to put more receivers in play.
Unfortunately for my beloved Tigers, the offensive line began to fall completely apart in fall camp with injury. Lots of people were shuffled around. Britt was a bright spot until he was slowed himself by injury.
Britt (#68) vs. Jarvis Jones (2012)
Mizzou began their disappointing 2012 campaign against Jarvis Jones and Georgia in early September. It did not go well. Jones was almost undoubtedly Britt's worst game at Mizzou. A couple things to notice.
1. Britt starts at RT but moves to LT just before halftime and finishes at LT. Mitch Morse had an insanely awful game snapping the ball and eventually had to move back out to his natural RT. The LT moved to C, with Britt moving to LT. Jones beats Britt on speed in that game, but nothing else. Missouri really doesn't shift protection or give much help, aside from the occasional chip from a RB. When they engage, Britt mostly locks Jones up. In a phone booth he easily pushes the smallish Jones around. Still, this was a clear win for the All-American Jones.
I want to stress, by way of comparison, how much help Alabama provides its vaunted OTs in their matchup versus Jones. Justin Britt is not in their category talent-wise (though his athleticism, as has been noted, is pretty damned good). The point is that Seattle plays a lot more like Alabama than like Missouri. (It's also that Jarvis Jones was SUCH a bad-ass in college... but you already knew that.)
2. This was Britt's worst game at Mizzou by a country mile, and it's far from embarrassing. Jones murdered Missouri with big plays in that game, but it's not like Britt had zero success against him either. Even in his worst tape you still see the things Cable probably likes about Britt, namely his play-through-the-whistle style and positional flexibility. Britt hustles down Jones (who had dropped into coverage) and keeps him out of the end zone after a red zone INT. He also moved from RT to LT in-game.
Britt vs. Florida
This is draftbreakdown's Sharrif Floyd tape vs. Mizzou. Britt and Floyd rarely match up in this game. Britt plays RT for the first couple series but then left the game. I want to point out how completely ineffective the LDE is against Britt, but then is able to bullrush the backup OT.
Britt vs Arizona State (2012)
This is draftbreakdown's Will Sutton tape vs. Mizzou. Again, Britt and Sutton rarely match up. Britt plays LT exclusively. RS Frosh, Corbin Berkstressor made a surprise start when James Franklin was hurt. (No one knew Franklin would not play until just before kickoff.) Again, note Britt's general consistency. He can struggle with speed but he is also routinely left on an island protecting QBs who hold onto the ball too long. His run blocking is pretty doggone good (see his get off from a 3 point stance in-line at about 4:15 on the goal line).
Britt vs Kentucky (2013)
This is draftbreakdown's Justin Britt tape vs. Kentucky this past season. You see the same things. Speed bothers him on the left side, but it's not a fatal flaw. I think his base is a little narrow and he's typically left on an island. Cable is, not surprisingly, looking at him primarily for the right side. If you value run blocking in your RT, which we know Cable does, what you'll notice on all of his tape is that he doesn't get stymied and pushed back. It's worth noting here also that Missouri ran the ball well against pretty much every defense this past season, save South Carolina. They finished second in the SEC in per carry average at 5.7 (to Auburn's 6.3).
Overall, Britt has enough flaws to raise legitimate questions about why Cable did not value other talents (e.g., Morgan Moses) more highly. At the same time, I can see what Cable likes about Britt. I think the kid can and will play. Now, I'd be a little surprised if he beats out a healthy Bowie. But, I also see this as Seattle hedging their bets on Russell Okung's health. If Okung cannot go I think Seattle wants the option of a kid who they feel comfortable playing on day one.