well, to simply call Dk an RTS is lazy.
There definitely is a difference first and foremost in that you are not in direct control of your minions in DK, as opposed to the right-click-to-the-death lemmings cranked out of a barracks in what Malk is positing as a "classical" build n' deploy RTS.
Traditional "build n' deploy" RTS lemming troops have no sim-like requirements either, such as "making them happy" for example. They have no personal needs beyond their prerequisite building material cost. Piss off your minions in Dk, and youll soon find yourself the commander of a grand militia of imps. To the contrary, a bread-and-butter atypical RTS lemming will simply stand there in formation, theoretically for all eternity, regardless of whether he likes smelly cavalry standing next to him, or has food available, or has entertainment provided. In Dk2, with what would in one form or another come to be known as "morale", some minions will even refuse to fight if they feel it's a losing battle or are exhausted. Or even if theyd rather go play in the casino or torture chamber a while, the polar opposite of the typical bread and butter RTS right-click-to-the-death lemmings that have no such behavior whatsoever.
I'd say that Dk is a sim/rts, not simply an RTS. That's selling it, and Peter short in my opinion. It's like Majesty, another great title that I feel falls into this category as well.