With an imported playthrough you can meet some of the people who you've met along the way and most of them will accompany you against the Reapers. There's also some choise that you've made in the past that affect your chances and possible choices on ME3 if you import a saved game. One of the biggest one that comes to mind is that if you saved Maelon's data on the genophage cure or not. Ummm... What else... Those are the ones that come to my mind quickly. I've only played with imported saves, though.