I know you wanted to distill one or two key reasons from your poll, but that isn't likely to happen.
Its all of the above for me, as well.
Like many, I got hooked on it because of the DnD jokes. (In fact, I found it after one was linked to from here.) The humour had me coming back, and I went through the archives and read them all.
Now, the humour is welcome, but isn't the main focus. I really do care about how this all wraps up, and what will happen after. I do hope that Haley and Elan end up together. I hope that Durkon gets back to his people. I hope we finally find out if V is a he she or it. And I can't wait to see if Roy lets himself be ressurected. (My guess is yes, since that means otherwise, he has to deal with his dear old dead dad for eternity...)
It was the first DnD in comic form I'd read. And while I haven't played much DnD of late, that also keep me drawn back to it. Believe it or not, I've learned a lot from OoTS as far as rules go. (Mainly in the form of: "Say what?" *flip, flip, flip* "Oooohh! Neat!")
The only thing that OoTS does not have is art. But then again, if it were really well drawn, it wouldn't be the "Order of the STICK" would it?