I always thought the main requirements for a professional camera were:

Manual mode

Plenty of accessories - interchangeable focussing screens, winders, databacks, interchangeable viewfinders on some models.

However, the lines are a bit blurred. Take the Pentax MV1. I doubt many pros bought them as they only offer aperture priority, 1/100 and B settings and don't even tell you what shutter speed they've selected (you just get a green light if it's 1/60 or higher). Yet it has the connections for a motor winder.