Only person I've had trouble finding is a healer, and not even that much trouble.
My Sorcerer is a healer, Trooper a tank, and Knight a DPS.
Also, it's tough trying to get rid of the "holy trinity".
City of Heroes (and Guild Wars 2 TBA) has also tried this, but then it only boils down to steamrolling the instance with little to no effort because everyone is a jack-of-all-trades.
Guild Wars 2 will probably end up being more facerolling because it's publisher (NCSoft, who insisted this mechanic) also developed City of Heroes. And we all know how that went.
P.S., I know the difference between developer and publisher, but the publisher has much more sway in the development process than you think