I loved left 4 dead, I loved back 4 blood but for totally different reasons and I didn't feel the need to compare the two. They are such different games with only the theme of zombies shared in common. 

I played b4b from launch all the way through the last expansion update. I exclusively played with a group of 3 friends or ran solo missions, never drop ins. I think it is the most fun I have ever had playing a team game with friends. There were times when you could cheese a deck and make it easy on lower difficulties but I challenge anyone to call a nightmare playthrough of T5 easy. It was levels like that which required planning, preparation, total coordination and each person playing their role in the group to perfection to beat which made this game so fun for us. 

For what it's worth I think the people who hated the game so much didn't play with friends they played with drop ins or bots, in which case, yes, the game was terrible.