I've been kinda thinking about this since the day it was posted. It's a great question, really.
What finally occurred to me is that I always logged back in because no matter what specific situation was really pissing me off, I always had 20 other things that I wanted to accomplish. I always had several items I wanted to check off on my to-do list, so I would refocus on something else for a while. Which would invariably lead me to more things, and more and more. And eventually that roadblock I couldn't overcome previously became incrementally more achievable. More levels, more allies, better gear, ... whatever the outcome of all the other things I pursued, the more able I was to conquer that other thing. And when I finally did, it was so sweet and so satisfying that it compelled me to tackle the next roadblock thru the same course.
And ultimately this is what I miss so much from other MMOs today. Either there is no list of things to accomplish because everything is on rails and there are no deviations from the prescribed path, or nothing is difficult to overcome to begin with. At best the only gate to success is time and repetition. Which generally results in me just being bored to the point that I just don't want to log in anymore.
In the end, the frustration from EQ was part of why it was compelling.
