Because it is a cropped sensor, with an 85mm lens, you cannot get the same framing as if it were on a FF body. To photograph a standing, six foot tall person with an 85mm lens on a 1.6x APS-C camera, you must have the camera 34 feet from the subject in order to get a field of view that is 8.47 feet tall, which allows some head-space and foot-space, and also allows for a slight bit of cropping. The facts are the facts, and the above is a fact.
The problem? At 34 feet, the camera to subject distance is so long, your backgrounds are going to be rendered relatively in-focus, especially at apertures where you need to shoot flash or flash fill-in. If tou are forced to do full-length portraits of people from close range with an APS-C body, ou need to resort to very short focal length lenses, in the 19-31mm focal length range, leading to exceptionally deep depth of field, with backgrounds that are very much recognizable.
Longer lenses help somewhat, but the smaller sensor and increased shooting distances of APS-C more than offset the focal length advantages of 85 to 100mm lenses. It's a vicious Catch-22. Still, if you need a lens suggestion for an APS-C, my favorite is Canon's 70-200 2.8 L IS--it has some of the nicest bokeh of any Canon lens, and handles well, focuses great (fast,reliable focus),has a fantastic stablizer system,and has good resale value. It simply delivers great images, shot after shot,with optical quality that is very "prime lens like". I see the name "Photography" in your sig; have you considered how awesome the original 5D is as a portrait camera?