It seems like a simple idea, but the diversity of Chapter is part of what I like about DW, over OW. For me, I like Cadians a lot, and think that the Catachans are just dumb as hell. The Elysians are pretty cool, if you don't have a powerful attachment to tanks, and most of the other Regiments don't do anything much for me. So, when I want to sit down and play OW, I have to hope that EVERYONE wants to be Cadian, or I might not enjoy it, as much. In DW, though, if I want to be an Ultramarine, or a Salamander, or an Iron Fist, I can be, and I don't have to be forced to play those Blood Angels and Space Wolves so many other people think are the only good Space Marines, while the Ultramarines are just vanilla.
Certainly, it opens one up to a certain amount of twinking and cliche choices, such as the Ultramarines Tactical, or the Iron Hands Techmarine, but I still appreciate it more than having to pick one Regiment, when an argument could be made that several are functional in a given warzone, and the Lord Militant controls each, at least to some degree. With the varying arguments I've heard for how Regiment most of the Specialists are (abilities and gear), compared to the Guardsmen, a certain amount of several regiments in the same team would seem fine, to me. It would give some appearance of a special unit, assembled by the Command for their specialties and gifts, and made to work together, rather than a group of Cadians, when one player wants to be a Storm Trooper (regiment sort of unimportant), another a Rough Rider (that's apparently a specific regiment, or two, now), the third a Heavy, and the fourth a tech-priest (see Storm Trooper).
To the main topic, no it would seem rather easy, over all. The rules should work the same, for the most part, and instead of taking orders from a Watch Commander or Inquisitor, it would be from your Chapter Master, or maybe some other head of a Company.