The problem with these generalizations is that leaders also debuff as much as controllers, tho for different purposes.
For that matter, many strikers and defenders have a sizable assortment of debuffs at their disposal.
This is an inevitable outocme of 4e's (initial) design, where building a class means coming up with, what, sixty or seventy powers distributed over thirty levels? And they're each supposed to be distinct from each other while also avoiding role overlap? Seems unlikely.
Other roles have some insulation from such intrusions by their class features, which give them an edge above and beyond their powers. This insulation isn't always adequate, as I've certainly seen fighters that can outdamage many a striker. However, controllers have the rawest deal as they, for the most part, lack a control class feature that ensures that they can "out-control" a leader or other role. To add insult to injury, they are generally saddled with bad AC and hit points around.
Basically, a controller ensures battle happens when and how the party wants it. There's often more monsters than party members, and it's impossible (and often fatally foolhardy) for a defender to cover them all. Basically, look at how your combats run. Do they tend to have monsters going willy nilly, often slipping in to flank characters that are out on their own? Or do they tend to have monsters in specific kill zones, with party members who require safety in safety? The difference is control. The controller makes it easier to manage monsters, rather than have the party be managed by monsters.
Well, are we talking about how it is, or how it should be?
As it is, the controller is just inchoate hodgepodge of powers that mostly daze and immobilize.
What should a controller be? My thought on the matter is this: a leader should be the guy who manages all the party's internal resoureces. This covers hit points, attacks, and defenses, among other things. The controller's domain should be over elements of the game that's extenral to the party, and this does not just mean "control" in the MMO notion of "crowd control" (dazing and immobilizing). Control can also be stuff like helping the party overcome an environmental hassle like a chasm or energy barrier.
Now, controllers can do some stuff like that. Most of them get free Ritual Casting, which is a step in the right direction. And wizards get the spellbook class feature. Beyond that though, the 4e approach to power allotment is pretty parsimonious. Sure, a controller might have a power that lets them create a bridge, but they need to have picked it, and if they picked it, that's locked in place until they can respec it out. A controller needs to be able to have the right tool for the right job, and I think that's not very well realized.