Number one is definitely the looks. But probably interpreted in a broad way. Yes, I find them aestetically pleasing to look at. But underlying the superficial appearance is an appreciation that they are healthy and organic.
Yes, I know that aquaria are by their very nature incredibly artificial constructs. But a considerable aspect of their allure for me is that they mimic natural systems.
I am far more drawn to the forest, ocean, mountains, or desert, than I am to cities, theater, or sporting events. Science, including biology and ecology, impress me as very real - moreso than any human constructs, and so tremendously necessary to our very existence. The efforts I put into maintaining a healthy ecosystem in my tank reinforces the importance to me of preserving clean environment on a larger scale.
That and I just like plants.
And I personally consider the equipment and chemistry somewhat of a pain - a necessary evil if you will.