It's always weird when a series that's "ok" suddenly finds a switch, turns it on and immediately becomes really good. The same thing happened to "The Americans" in season 2, after season 1 was entertaining yet not really all that special. It also happens with Halt and Catch Fire. The show exuberates a new-found confidence, using it in interesting ways and taking the story and the characters to places it didn't dare before. The result is a resounding success.
Joe, for example, still remains a cipher, but this is now acknowledged by everyone and the show, and everything in his arc revolves around it. He is a Steve Jobs variant, including beard and glasses, and flabbergasting his hires, his board and his former friends alike, let alone the industry at large. He remains aloof, and when the show peels him back layer by layer, exposing him with excruciating slowness, the result is worth watching and ends with a barebones salesman pleading with his estranged friend in the basement server room.
Gordon finally gets real. He suffers from the lack of appreciation and outright disrespect at the workplace, and he suffers from Donna drifting away. Trying to be a good and supportive husband, he cannot but lapse back into former behaviors, trying to be the genius he always wanted to be (and that he was sold at in season 1), but never manages to. Rather, he's competent in a certain area, and he has serious social interaction issues. Instead of burying that under Sorkineseque genius bullshit, the show finally acknowledges it. As a result, the character can grow, and in the end, he is a man transformed, and at peace with himself.
Cameron is positioned right on the divide between doing what she loves - coding away problems of her labor of love - and the necessities of managment. She desperately tries to keep her creation hers while it becomes too succesful, too big to handle in that way, and her arc tragedly winds itself to the inevitable confusion of the loss of her company, while taking turns and deviations. The series settled on Cameron being the true genius, and instead of everyone fawning over it, people are put off, and Cameron's arrogance costs her dearly. In the end, she is still not in a happy place, but she sees a path there.
Donna is the exact opposite of Cameron. Without the technical skills or the genius of Gordon or Cameron especially, she has to channel her inner Joe and embrace the business side. Donna is going to some dark places, and she seeks out success in the business world, with Mutiny being more and more the expedient tool to do this rather than her brain child, which leads to inevitable clashes and losses along the way. She ends the series in the most unhappy place, her newfound sense of business and shark like sense of weaknesses and opportunities leaving her alone. She is, for me, the most interesting character of the whole thing, and given where she started, that's quite a feat.
The series also does some interesting structural things. For example, the time-jump between episode 8 and 9 of over four years without any warning and just expecting that the viewer rapidly adapts to the new status quo is great, and a lot of stuff can be done with it. It shows you what a confident show with a great writer staff and characters can do.
The whole season also salvages a lot from the earlier two, using scenes and character arcs to great effect (like callbacks to Comdex in season 1), banking on nostalgia and giving a distinct and believable feeling that these characters aged, and that their past becomes more and more like baggage, a development sadly missing from season 2.
All those arcs are complemented by stellar performances of everyone involved. The visuals, camerwork etc. remain top-notch, and so, craftsmanship and writing finally come together to create something great. Halt and Catch Fire, in its third season, really is worth the time.