The plot was the main thing that attracted me to it. It's vastly different from any series that I know of (except Weeds from what I've read). I also love how open ended it is. It's completely unpredictable and, right now, I've absolutely no idea which direction the end of season 5 will go.
The cast is great and varied and I love Walt's and Jesse's relationship. Sure, it's toned down now, but in the early seasons they argued and fought like cat and dog, almost polar opposites, but they still have each other's back in life/death situations.
The transformation of Walt has been showcased brilliantly. A chemistry genius, from struggling as a high school teacher to a feared meth kingpin is fascinating when you watch it through. His attitude, morals - everything - changes hugely but you always remember that it derived from his love for his family. His primary goal after all was to secure finance for his family when he dies.
Finally, I enjoy the comedy aspect of it. No way is it to be viewed as a comedy, but there are overtones of black comedy. Also, Hank Schrader's and Saul Goodman's use as comic relief is great and it's not done in excess.
