I read that the boss battles are so difficult in this game that you are encouraged to play them (co-op) with the help of a friend, but I was able to complete all 33 main missions without help and I'm not the greatest gamer. But I am no stranger to MGS and have played all the games, I even bought a gamecube just to play Twin Snakes, and I think this game is a great addition but possibly not the most accessible. The game has no easy level that I can see and it has a somewhat confusing rpg element.
Throughout the game you use the fulton recovery system, a balloon, to send incapacitated soldiers and released prisoners back to your home base and these people form your team to make your weapons, supply your intelligence and become your army. C4 just doesn't magically appear you have to get your team to make it. So the more team members you have and the smarter they are the better weapons you get. Also, you can send out soldiers and tank divisions to fight battles and bring back rewards, but to get a tank division you are going to have to play the same level where you capture a single tank over and over again until you have enough. That is what is so great about this game you can play the same level 20 times or more and it helps you advance in the game, you get more people and tanks and your weapons get leveled up. That is why this game was 60 hours for me and there are still some extra missions (there are 128 extra missions in this game) that I haven't played may never play.
Another great thing, most missions are about 15 minutes long unless you get killed four times in a row, perfect for when you got a few spare moments. I also liked that the game stopped just before a boss battle so that you could pick the best weapons and gear for the fight/mission. When I had trouble with game I checked the guys on youtube to get a few ideas. Now I can't wait for Metal Gear Solid Rising on the PS3.