I didn't participate much (and whatever hats I collected happened by accident) but I liked it, and it was very well designed and executed. One thing though - some of the hats encouraged "randomly click the vote button twenty times" type behaviour [that had caused problems earlier this winter][1]. I'd rather if next year's winter bash dropped those hats that could be gained that way.

Other than that, I'm looking forward to seeing it again in twelve months' time.

Also, I'd like to see one outside-the-season hat - just one, and limited to a year per holder - for some really, really special services to the site, say faithful retagging, editing, and other activities that are vital to the site, but don't earn you much rep. But that's a different story. 

  [1]: http://meta.stackoverflow.com/questions/155561/the-robo-approvers-are-killing-my-will-to-review-edits