This is how a season finale should be. Tension the entire time and then something crazy happens. And then after the craziness, a complete bomb is dropped. This finale almost made me have a heart-attack.
Quick recap of what happens: Walt hatches a plan to kill Gus. He uses ol' Tio (the bell-ringer in the wheelchair), he gets Tio to go to the DEA and basically mess with them. This causes Gus to pay Tio a visit (to murder him) and Walt plants the bomb on Tio's chair and the bell is the detonator. The look on Tio's face when he hit's the button is pretty awesome.
They show the outside of the room and Gus comes walking out. At this point, I was irritated that he somehow survived the blast. As much of an awesome villain as he is, I cannot imagine a way out for Walt and Jesse other than his death (or Walt going to Hank and "admitting" everything). Then, Gus turns and half his face hi blown up. He drops dead. Or you can just watch this GIF over and over.
The police have been holding Jesse because of his ricin comment and then they release him because the poison turned out not to be ricin at all, but instead it was Lily of the Valley, a common New Mexico plant that has poisonous berries that children sometimes eat.
As Jesse leaves, he is grabbed by Gus' men and we see him working at the lab under gunpoint. Then the lab opens up and it is the other guy, but he is dead and Walt appears from behind and shoots the guy. Walt and Jesse get to work destroying the lab.
Jesse explains to Walter what the doctors told him about Brock. Jesse seems conflicted about killing Gus, but he is trying to convince himself. Then they show Walt's chair by the pool and there is a zoom in of a plant, and you see the tag and it says "Lily of the Valley." So Walt really did poison Brock. He crossed a line and has completely went to a new place.
So where does that leave us for next season? Well, obviously Hank will not give up on Heisenberg. I think that he will eventually suspect Walter. What would Walt do to protect his secret? Would he murder Hank?
Where and who will they cook for? Obviously now that Walter has a taste for the money, he will want to continue cooking meth. Actually it is not about the money, Walter likes the power, it reminds him of what he gave up as a great chemist.
I only see this ending one way actually: Walter dying, most likely from cancer.