Design is king
Some of the in-mission improvements from the original game that I really, really love are such things as being able to stand thirty-plus feet away from a door, or whatever, and tell my men what to do with it i.e. open and clear (burst in and kill everyone) or open, grenade, and clear and so on (generally while I'm hiding safely round the corner having a quick smoke). This just makes the game so much more enjoyable than having to do all of it by yourself. Because you only get to set up three teams in the planning phase - red, green, and gold, you'll quite often find that you could really do with another set of guys to patch a hole in the plan, and due to the ability to directly control your team (and to a limited degree the other teams as well - you can switch between teams at will) it's a bit like having a fourth team made up of just you. You can, for example, get your guys to cover an arc of fire from a doorway while you nip round the back and secure the area from a different angle, so that you are covering each other in a truly effective manner. This kind of thing is really quite important in a game like this as one of the really huge things about all of the R6 games is that they have random placement of bad guys each time you play a mission - that's to say, some of the bad guys (in areas vital to the mission, for example) tend to be in roughly the same places each time - but most of them get jiggled around a bit, and not just in their placement but in their movement habits as well: sometimes a guy will walk through a door, and sometimes he won't. So you could be waiting for a hell of a long time for someone who just doesn't show.
Or not. That's the great thing - you never know, so the step-counters and the patrol-route-memorisers and so on are finally going to have to work for a living. I don't know if it's just that I'm thick (and no answer on a postcard please) but I have to do these missions a lot of times before I get it just right - especially on missions involving hostages because sometimes they shoot the hostages, and sometimes they don't, despite the fact that I do exactly the same thing each time - so you spend about ten goes setting up a system to prevent the murder of a hostage and just when you've got it right, the bastards suddenly decide that they're going to kill him anyway: frustrating, frustrating, frustrating. But then that's also part of the fun. You can see this in the fact that it's not possible to save a mission partway through so every time you play you have a real sense of tension which was the defence case put by the designers of Project IGI when criticised for this, and they are right, but that doesn't make it any less irritating to me.