I'm going to guess that other 30% is split between classes which play closer to Solider but has some interesting powers and I'm going to guess that's Vanguard and Infiltrator about covers the rest. You are going to get all you are going to need out of the class system playing those 3 classes. You play through it once as Solider, than you might want to use some of the tech or force powers so you pick a class which plays closer to Solider anyway. It's so funny really, most people play this game as a shooter. In fact, they put action option in ME3 because most people SKIP all the dialogue and just want to get straight to the gameplay. Quite the inverse of what people on the forums tell it's all about the story, characters and choices they are going to remember forever and that jizz.