I agree with those that say World Cup performances shouldn't be the defining factor, but I really don't get how people can disregard it entirely simply due to the rise of Champions League football. Injuries, mismanagement and simple bad luck can play hugely important roles in tournaments that only last a month, yes, but it's that added challenge that makes it such a commendable achievement. Ronaldo overcame injuries to play a starring role in the World Cup. Zidane overcame mismanagement to play a starring role in a World Cup. At the end of the day, all of the great players faced the exact same problems and it's how they dealt with them that allowed them to play a decisive role in one of the biggest sporting events around and in many cases the pinnacle of their sporting careers that establishes them as truly a level above their peers.
The thing is, the people that are arguing against the World Cup being an important factor seemingly always argue against the most extreme version of the opposing argument - arguing against the idea that he needs to stand head and shoulders above everyone else in the tournament and almost single-handedly drag an average team to win it. There are so few people that think this that it's more or less a straw man argument. Cruyff didn't win the World Cup but his performances in '74 are still right up there with his greatest achievements and at the heart of his legendary status, and if Messi had done something similar I've no doubt much of the criticism would've died away. I'm not saying he should be expected to perform at the same level because Cruyff played with a group of players he was extremely familiar with and they were all on form, and that's not true of Messi, but the point is winning the tournament is not the be all and end all. It's just about showing something close to your best form under a different set of conditions. That's hardly a unique challenge that only Messi has had to live up to.
If you're talking about the great players you're talking about Maradona, Pelé, Cruyff, Beckenbauer, Puskás, Di Stefano, Zidane, Ronaldo, Best, Garrincha, Eusébio, Platini etc. and the only player there that played in a World Cup and didn't show something close to his best form was Platini, and it's no doubt partly because of that that he's considered a tier below many of the others. Best and Di Stéfano didn't get a chance so that's not something you can hold against them, but the rest all have at least one really memorable World Cup performance to point to. I don't think Messi's performances last year can compete with any of the others and I don't think it was any better than Platini in '82, but his World Cup performances are almost dismissed entirely. Platini of course has his 9-goal haul in Euro '84 to point to which is a level above anything Messi's produced in an international tournament too. You can say that they all played in better teams so obviously they played better individually, but for me there's not much difference between Maradona's Argentina, Zidane's France and Messi's Argentina. They were solid, functional teams who were extremely reliant on their talisman to win them the game. The first two coped with that better than Messi did, IMO.