I was going to ask the same thing, the ranger does seem to do some of the things you would expect a controller to do, besides it always seemed silly to me that they would have two martial strikers in the first book, but only 1 controller, makes me wonder if they originally intended the ranger to be the second controller.
Ironically it basically does what I would think a martial (or any non-casting controller), would do, basically use speed, multiple and area attacks to hamper the enemy, plus the fact that many of its powers can be used at range or in melee.
Really the only things I think are missing to make it a full blown controller is battlefield altering effects, I could easily see these done through pinning effects type abilities
Maybe by giving it interrupt powers that allow the ranger to move to and attack opponents for melee or make ranged attacks against opponents who move within short range and LOS, or attack an opponent, secondary effect burst 5 vs Will, effected opponents cannot move or maybe lose an action instead, ie minor, move, or attack), maybe utility powers that allow the ranger to set traps prior to battle, or maybe even attack powers, as long as they aren't too over the top
Right now, the assumption of a balanced party means every group has to have a Wizard, though, which kind of sucks