The top 5 passing attacks from last year was, in order, NO, NE, GB, DET, and NYG. All of these teams have considerable production from their slot WRs except Detroit. Lets take a look at the weapons these teams have and how they utilized them.
Slot Stats: 19% of total team receptions and 23% of total team yardage
The Saints use a 4 WR set regularly with their 3rd and 4th WR getting at least 32 receptions. Saints main Slot WR is Lance Moore(52 rec, 627 Yards). The catch with this team is that they also use Darren Sproles(86, 710) to attack the middle and even work downfield.
Slot Stats: 30% of total team receptions and 30% of total team yardage
The Pats never really used a 4 WR set last year. Their 3rd WR was Chad Ochocinco with 15 receptions. Their RBs also don't catch a lot of passes either, however their TEs caught 42% of their receptions with 43% of their yardage. Rob Gronkowski gives them a pseudo 3 WR set whenever he's on the field allowing Wes Welker(122, 1569) to work the middle and underneath routes.
Slot Stats: 27% of total team receptions and 28% of total team yardage
The Packers also used a 4 WR set regularly with their 3rd, 4th, and 5th WR all getting at least 25 receptions. They have the highest use of WR of the top 5 as 63% of overall team receptions went to their wideouts. They also use Jermichael Finley to effectively attack the seams downfield.
Slot Stats: 11% of total team receptions and 12% of total team yardage.
The Lions mainly used Nate Burleson as their Slot WR in 3 WR packages. They also got zero production from their 4th WR, Rashied Davis(4, 63). Early in the year they were able to use Best to work underneath and also downfield.
Slot Stats: 23% of total team receptions and 31% of total team yardage
The Giants didn't get any real production from their 4th WR. They mainly relied on Hakeem Nicks, Mario Manningham, and Victor Cruz. Cruz(82, 1536) was their man slot WR, with the best production of the WRs.
Note: Slot Stats are based on intuitive analysis of team statistics.
We can see that all these teams had explosive and productive players as slot WR. These teams used their slot WR to open up the outside receivers, while the Lions were the opposite using Calvin to open up the slot. It is interesting to see that they all used different WR, RB, and TE combinations to get the job done. New Orleans with their RBs, New England with their TEs, and Green Bay with their outside WRs. The Giants and Lions were the most similar from a production stand point, and also by not really getting any help from a 4th receiving threat. But as the numbers indicate the Lions were well behind in slot WR production.
Before last year everyone was looking to upgrade the Lions 3rd WR, as Bryant Johnson was terrible in this role. They needed a vertical threat opposite of Calvin to "take the top off" the defense. Enter Titus Young. Now this year they took another WR in Ryan Broyles to upgrade the slot WR on this team, and probably go to more 4 WR sets. I think this could potentially make the Lions unstoppable. They could conceivably go with a New Orleans attack with 4 WR and a RB, or a New England attack with 2 WR and 2 TE, or a Green Bay attack with 4 WR and a TE. The options this offense now has to exploit any weakness a defense has is incredible. Any coverage deficiencies offer the Lions a mismatch, whether at linebacker, corner, or safety. The pick of Broyles could be the most important pick they made this year.