Laura Miller Opts Out
All I can say is wow. Laura Miller's not exactly won rave reviews for her tenure (or style) and Dallas can be a rather divisive town to try and govern. But still, I would have guessed she could have cruised to re-election.
The DMN gives an overview of some of the contenders for the gig now ... I can't say I'm well versed in the personalities covered there.

First, let me say I mostly like Laura Miller: I think her intentions are often good, and she's been a lot better thank Kirk (from a surbanite looking in). I thought her efforts to bring a strong-mayor government was exactly the right thing, and I think reading the rejection of that proposal as a commentary on Miller has some merit.
But she was looking at a tough re-election that, though she might have won, could well have left her weaker as a mayor or at least led to a term of more squabbling and foot dragging. The police and fire departments would probably not have endorsed as they did before, and the screw up with the Cowboys has more than a few folks not so much mad at her for not getting the Cowboys back to Dallas as demonstrating a lack of planning and aggressiveness that a city like Dallas should have. (And I can't say that I disagree: Addison has a better sense of planning and identity than Dallas. Bringing the Cowboys back to Dallas could well have rejunvenated the Cotton Bowl and that part of Dallas, one of the few times where I thought city financing of a sports team was worth considering.)
I can't think of an opponent that would have challenged Laura on his or her own. The sad thing is that, though I'm not up on all the Dallas council members and possible mayoral candidates, I don't think any will have Miller's leadership, as flawed as it was at times.
Finally, Laura had cancer once upon a time, and this is one person that I believe truly wants to spend time with her family. (I'm not saying she has it again, just that I think she has a good perspective on family and work.)
FWIW, I'll be surprised if racial tensions aren't simmering in the background of the next mayoral election. It'll be interesting to see if any candidate can rise above the north Dallas vs south Dallas politics.