I started reading like, two years ago or something. I'm kinda wondering if I was slightly retarded or something at the time, but I couldn't figure out how to get to the next page/continue the 'game'!
Then again, it was quite a few years ago... perhaps only one page existed at the time and there was no '==>' button!
But yeah, anyway, quite a long time later, I started seeing troll pictures around and decided to check it out and was all "What the hell-- I remember this confusing shit! I'm just gonna get out of here-- oh. A button."
And I kept reading.
What drew me in is how Andrew used all of the elements that, at first, seemed like totally pointless crap! The endless Chekhov's Guns amazed me.
And then. Oh god, and then. The flashes started getting more and more advanced. Oh goodness me~! *fans my face*
So in short, I came for the plot, I stayed for the art.
The first time I found Homestuck, I was following a link from a pretty cool pic of the kids on DA. I didn't get past the first page 'cuz it didn't look like my cup of tea (I'm horrible and judge everything by its first sentence.) Then I went to Sakuracon and saw all of these weird grey Zodiac people. Having an obsession with all things Zodiac, I asked one of them what she was from and she replied "Homestuck."
I went back to the sight and was surprised to see I'd already been, but this time I read on to the second and third page. Somewhere within those first few I got hooked, and each successive flash video made it even better.
I got linked to Homestuck from some other forum (a Bionicle fansite) and decided to give it a shot. Although I was a bit disappointed upon the realization that it wasn't actually a text-based adventure game, that disappointment faded within a few seconds as John did silly things in his room with witty commentary. The first dozen pages or so might have seemed like a chore, but as I continued reading it just got more intriguing and more humorous. I became genuinely interested in finding out what John did next.
Several more pages passed, and eventually I made it to the joke contained in the transition between this page and this page. Something about that one -- the timing of it, the unchanging perspective and expression between the two panels, the accompanying "You figure you've left him hanging long enough" -- just made laugh hysterically. I'm pretty sure it was at that point that I decided I wanted to see this comic through until the end.
So the humor is mainly what drew me in, I guess. Then as kids started entering the Medium, the plot started to get more interesting, and now I'd say I'm "stuck on Homestuck" for both the humor and the crazy story.