I personally find ME3 too linear. You have all your main missions that must be done one after another, ducks in a row. You can do side stuff to spread them out (though side stuff amounts to "play multiplayer map"), but the order you must do them in does not change at all.
ME2 spoiled me, with it's surprisingly open-ended "plot" (considering the recruitment and loyalty missions as plot) mission options.
Don't get me wrong, I enjoyed ME3 immensely. But I find it hard to replay mainly because of this: there's really very little semblance of an open world (the stupid Act format is a part of it), and the plot is completely linear. And, for that matter, just the whole theme of urgency is not one I particularly care for over an entire game. Over parts of it, sure, but over an entire one it creates that linear experience that makes me enjoy exploring less (I feel like I have to rush). A comparable game in this regard would be Lightning Returns: Final Fantasy XIII.