Oh Lethean, shit will get real real soon real fast, and I'm guessing you're going to like it.
My interpretation of his spiraling downfall is that Don is essentially a metaphor of the decadence of the american dream, the impossibility of ever achieving it, and is, in essence, an emblem of the meaninglessness of advertising and its promise, which is happiness.
It's also worth noting that post-modernism was all the rage in the 60s, and it was at that time when post-structuralists introduced the notion of "The medium is the message", which in a nutshell asserts that it's not so much what you are saying it, but rather how; and the mechanisms of the how carry a far more significant connection with the observer than the message itself. This was also a time when subliminal advertising was used.
In short, Don's downfall (as witnessed by his literal fall in the opening credits, surrounded by ads) is a representation of advertising's inability to truly deliver happiness and instead it smothers the consumers with things they don't need. Don's personal downfall is reflected in this by his own inability of ever becoming happy, despite having everything he's ever wanted...something always seems to be missing, and he can't help destroying his relationships with people close to him by trying to find it.
Jon Hamm's portrayal of the nuances of complexity found in Don's character are nothing short of some of the best acting I have ever seen. Truly, a fantastic show, and I'm glad there are others here who watch it.
