I'm trying to work on my query still, but what I've learned is this. By far, the most popular tag for first time users to this site is story-identification. Note that I make no difference between questions and answers with my query. The second column shows the average number of posts per user, including the first one. Looks like Story-Identification doesn't do as well there, although there are worse subjects. Bottom line is, they seem to bring people to this site, but most of them don't stick around for very long.
tagname Users PostsPerUser
---------------------- ----- ----------------
story-identification 627 2.242424242424
books 112 2.339285714285
movie 98 5.551020408163
short-stories 98 1.877551020408
star-trek 92 11.858695652173
star-wars 85 8.658823529411
harry-potter 81 4.975308641975
aliens 73 2.082191780821
novel 71 3.422535211267
time-travel 61 6.081967213114
futurama 54 2.166666666666
lord-of-the-rings 53 3.735849056603
In addition, I made this query to filter out cases in which a user only asked/answered one question.
tagname Users PostsPerUser
---------------------- ----- ---------------
story-identification 148 6.263513513513
star-trek 50 20.98
star-wars 42 16.5
movie 36 13.388888888888
harry-potter 35 10.2
novel 26 7.615384615384
short-stories 26 4.307692307692
lord-of-the-rings 26 6.576923076923
time-travel 24 13.916666666666
books 23 7.521739130434
suggested-order 18 23.222222222222
star-trek-tng 17 38.35294117647
tv 15 12.666666666666
doctor-who 15 11.4
magic 14 5.857142857142
a-song-of-ice-and-fire 14 8.214285714285
futurama 14 5.5
george-r-r-martin 13 8.461538461538
stargate 12 4.166666666666
aliens 12 7.583333333333
comics 11 9.818181818181