Going into the offseason, I would have (like many) ranked WR as the #1 weakness on the Viking roster. By swapping Percy Harvin for Greg Jennings, I don't think that is the case anymore. I am not saying that Jennings is as good a football player as Harvin or that he can do as much for an offense. But I've long viewed Harvin as an undefined, unique RB/WR hybrid rather than a real wide receiver (I've usually called him a glorified third down back, but that's not quite right). He's a great football player, but he did not provide the Vikings a downfield passing threat. Replacing Harvin with Jennings may not be a better move for the offense, but it does make the wide receiver unit itself better.
I would have ranked the #2 weakness quarterback depth. After signing Matt Cassel, I don't think that's the case anymore. Oh, you could still argue that quarterback itself is the team's biggest weakness--you might have a case. But there is no longer such a gigantic dropoff after Christian Ponder. If Ponder is injured or ineffective, the Vikes could basically run the same offense with Matt Cassel. The position is still all about Ponder, but there's no longer such a glaring need at backup. At backup QB you can either go with young project or experienced veteran, and if you're going with experienced veteran, is there a better backup option than Cassel? Kyle Orton? Ryan Fitzpatrick? That's the world you're talking about at backup QB.
So after the additions and subtractions, as of this very moment, what are the biggest weaknesses on this roster? Where is a team with six picks in the first four rounds going to fill in players that may need to start immediately?
Cornerback/Secondary. It is always--always--a need, and the loss of Antoine Winfield makes it a glaring need. If the roster doesn't change, it appears, what, Chris Cook and Josh Robinson start? OK, there's some potential there, but you can't be sure how good that combo would be. Robinson is unproven, and plenty of CBs who show potential as rookies get burned as sophomores. And who is the third CB on the field, which in today's NFL is quite an important position? How disastrous would the situation be if a starter gets injured? And couldn't the team use a legitimate upgrade at safety opposite Harrison Smith? We're still facing the likes of Aaron Rodgers and Calvin Johnson regularly, right?
Middle Linebacker. This is a pretty obvious point: Jasper Brinkley started 16 games at middle linebacker last season and he's not on the roster anymore. Audie Cole was second on the depth chart. Presumably Chad Greenway and Erin Henderson are asked to do the same things they did last year. Is Audie Cole ready to start? Is somebody else? Do you even need a middle linebacker in this defense?
After that, the Vikes can largely target the proverbial best player available. Teams always have to be adding offensive and defensive linemen, and there's no reason to ignore the wide receiver position (the Vikes could seriously seek out another starter at the position, including in the first round). But outside of these holes, the Vikings simply need to add talent to the roster and be willing to find potential starters at any position.