I'm not saying you didn't want a crossover. I'm just saying you probably didn't want it like this:
Instead, characters from Left 4 Dead 2 will exclusively show up in the Resident Evil 6 PC release, and characters from RE6 will exclusively show up in the PC version of L4D2.
RE6 players will be able to play as L4D2 characters Coach, Ellis, Nick, or Rochelle in the RE6 mode exclusive to the PC, The Mercenaries No Mercy. Making all of your L4D2 nightmares come true, the Witch and Mini Tank will also make an appearance and pester your characters just like the good old days. In turn, RE6 enemies will appear in L4D2, which Valve developers made possible with Steam Workshop.
So basically, it's the equivalent of Nick Fury showing up after the end credits, without the promise of future crossover goodness that entailed?