I liked Liesje's "OMG I'm getting a dog!!!" and then "OMG I have a dog!!!!" and then "OMG I have TWO dogs!!!!!" threads. 
Bunches of little idoggers born and conceived! 
A few super drama-tastic threads, many of the most memorable ones started by the same few people, which always struck me as a bit more than coincidental!
The pet food recalls were terrible, but you guys were so great keeping everyone up-to-date with the latest info.
Those were some of the most memorable moments for me.