Over at Crosscut, Knute Berger says that the mayor's race is all about style, and yeah, I suppose there's that. Ed Murray's campaign has certainly wanted to make it about style—because Mayor Mike McGinn is mean or something—and with so few substantive issues actually at stake, style has been the primary focus of the insider chatter.
So yeah, I'll give Berger that. And his thing about Seattle's "split personality."
But I think there's actually a simpler explanation of the dynamics of this race, and that is that McGinn isn't actually a Democrat. Oh sure, he votes Democratic. And in words and actions he reflects the values of Seattle's rank and file Democrats at least as well as Murray. But he's not a party Democrat. He's not an establishment Democrat. And that's why much of the Democratic establishment resents him.
Values aside, McGinn ran for mayor four years ago as a political outsider. He never worked his way up through the party machinery the way you're supposed to, putting in tedious hours at the LD and other levels. In fact, he'd totally fucked the party establishment just a couple years before when he spearheaded the charge against the party-backed Roads & Transit measure. A lot of Dems have never forgiven McGinn for that, even those who acknowledge that we ended up passing a better transit-only measure a year later. Hell, even I had trouble forgiving McGinn for what I still believe was a too-risky gamble.
Murray, on the other hand, is a party animal! He put in his time and worked his way to the top the right way. (The fact that his one year at the top of the Senate Dems was a total fucking disaster, well, that's not important.) There really isn't much of a Democratic Party machine in Seattle compared to other big cities, but a Murray victory would represent a reassertion of what authority it has, as well as a repudiation of that usurper McGinn and his wretched outsiderness.
To be clear, I'm not suggesting this dynamic as an explanation for the outcome of the race—most voters are only influenced by it indirectly. But it does help explain the irrational eye-rolling hatred of McGinn I've heard voiced from many Democratic insiders whose political judgement I otherwise trust.