A few aspects mentioned in your question will be our starting point. Please note, we are not saying each of these issues will be determining factors for every photographer. We're not saying one system is better than the other because of... a or b. Rather they are a response to the question, "...what all should a person consider?" Once considered, each of these aspects may or may not lead a particular portraitist to choose one system over the other.

• Focus System You may not need a super fast, complex, and configurable focus system like a sports photographer does, but you do need one that is consistently accurate unless you are manually focusing all of your shots. Unless you are shooting tethered and on a tripod, just seeing the scene well enough to manually focus a mirrorless camera can be a challenge. The lenses are designed around the assumption of AF and may be difficult to adjust in finer increments. Each camera/lens' shot to shot variation also plays a big role when you are shooting at the wide apertures typically used in current portraiture. Is the Mirrorless Interchangeable Lens Camera (MILC) you are considering consistent re: focus accuracy from shot to shot? Are the lenses as stable as heavier DSLR type lenses in terms of holding focus? Will using heavy DSLR lenses via an adapter on the MILC increase the risk of tweaking the mount?
• Dedicated Buttons This really depends on your shooting style. A lot of portraitists use a back body button to meter and/or focus, or a dial or joystick to select various focus points. If doing environmental portraits, the conditions can change every bit as fast as those at a wedding, since many wedding photos are essentially environmental portraits.
• High ISO Capabilities This also depends a lot on what types of portraits you shoot. It is not much of a factor in controlled studio lighting or outdoor daylight shoots, but what about outdoor night settings or other environmental types of portraits when you want to balance the dim ambient lighting with your controlled lighting?
• Long Telephoto Lenses Lenses beyond 200mm are not usually a factor when doing portraiture, but for tight facial framing 135mm or even 180-200mm is not unreasonable. And to get that smooth bokeh your customer expects, you are going to need a fairly fast telephoto lens. If there is even a 180mm f/2 equivalent MILC lens available, it will be about the same size/weight as the DSLR counterparts, so where is the MILC advantage?
• Resolution Sharpness is much more about the glass than the sensor most of the time. You can put the same 20+ MP FF sensor in a MILC, but if you don't have glass just as good, the DSLR with better glass will still outperform it. It is often possible to use "Pro" lenses on a MILC via adapter, but the added size and weight of those lenses remove the biggest advantage of the MILC systems.
• Ruggedness If you do outdoor portraits or other types of environmental portraiture, weather sealing and environmental resistance should be a consideration. Most pro level DSLR bodies and lenses, and even consumer level models such as a few by Pentax, are much more capable in this area than the current crop of mirrorless interchangeable lens cameras (MILC) on the market. This is especially true in terms of the lenses made specifically for the mirrorless systems. And while it is often possible to use "Pro" lenses on a MILC via adapter, the added size and weight of those lenses remove the biggest advantage of the MILC systems. In terms of body durability and ruggedness, at this point in the game there is not much comparison between the Nikon D4 or Canon 1D X and the top MILCs. You can take those pro bodies and use them in places that would bring the MILCs to their knees, if not totally destroy them. Not an issue if you are in the studio, but a big one if you are doing environmental portraits in extreme environments.
• Lens Selection It only takes one lens to make a portrait. But if you need a lens to do something none of the lenses designed for MILC systems can, there is almost certainly a Canon/Nikon/Sony/Pentax/Zeiss lens that can. As we've already repeated, using the full size lenses on a MILC removes one of the significant advantages of the MILC concept.
• Strobe Compatibility Another aspect to consider is how capable a MILC is at controlling multiple off-camera flashes via the camera's menu. This is something the Nikon CLS does very well. Canon bodies also have a lot of capability in this area and are gaining ground on Nikon with the release of the radio controlled (vs. optically controlled) 600EX-RT. Other full size DSLRs also do this to one degree or the other. This is something every "location" portrait photographer should consider essential. Can the MILC system you are considering do this?
• Bokeh To get the creamiest bokeh you need a large sensor and a fast lens with smooth aperture blades. Can your MILC lenses do that? There are a few that can, and a few new MILC systems include a FF sensor. But can any MILC system lens touch the results of using a lens such as the Canon EF 85mm f/1.2 L II?
• Tethering Since tethering is related to firmware and software compatibility, it shouldn't take much for manufacturers to include this capability for MILC systems that don't already have it. The question, though, is will the marketplace demand it?