Originally, Catalina's hairstyle was meant simply to be cat-like, and while Dan figured it would be somehow possible as a hairstyle in reality, he didn't quite know how it would work. A bit of fan art, however, helped with that.
According to Dan, this is why Carol wound up related to Sarah as part of a Retcon.
Dan suggests that Nanase's "magic boobs" are a result of magic affecting physiology after many people questioned how she could be so well-endowed despite her athleticism.
Fan Nickname: Several, mostly relating to bent gender forms.
Man Nanase is Manase, for instance.
Also an authorial nickname for Assistant Director Liefeld. Author's notes refer to him as "Muscle McMuscles".
Ellen's cat form has garnered the nickname "Katelyn".
Some parts of the fandom have named Grace, Gracie Sue.
The arc villain of the Family Tree storyline is called not-Tengu by the fans due to the fact that Tengu was just his online handle and no one in-universe was interested in hearing what his real name was, cutting of all his attempts to give his name.
Ashley who has a crush on Elliot is given the nickname Ponytail Girl and variations thereof due to her distinctive side ponytail, and the fact that it took a while before readers learned her name.
I Knew It: There is a section on the fan speculation page with confirmed or partially confirmed pages. Dan Shive has also mentioned in commentaries that he received messages from fans that correctly guessed certain plot points on several instances.
Jossed: Dan Shive, has done this too many times to count. In fact, the comic formerly seen on the trope page is a filler sketch made in response to a massive in flux of crazy and just plain wrong theories.
Schedule Slip: For a very long time, Dan was all over the place, barely squeezing out two strips a week. Grace's birthday party was one of the worst. He eventually started falling into "No set schedule, but frequent updates," but eventually started updating every weekday (sometimes skipping Wednesday if it's a bad week).
After big storylines, Shive often does a few strips detailing the planning process. After Painted Black, he noted that originally it was going to be Vlad hunting Grace instead of Hedge, leaving a trail of corpses across the countryside. Considering that the arc is already one of the least favorite among the fandom due to Cerebus Syndrome, you can understand why he didn't want to make it even darker.
Dan frequently refers to what could have happened had he done the uniforms plot as a briefer standalone arc instead of a B-plot that ultimately lasted five years.
Word of God: Dan Shive has often answered questions through Q&A comics. He also used to do so through the forum but has since ceased posting because of repetitive, predictable complaining. He has started answering questions via formspring but seems to be very selective about which questions he answers. On the other hand he tweets liberally about El Goonish Shive, often using the hashtag #EGS.